This study was planned to study the effect of L-arginine on some reproductive functions in male rats.The study was carried out at the Department Of Physiology, Faculty of Veterinary Medicine, Zagazig University.This study divided into two experiments as the following:Experiment I:-60 healthy male rats weighing 250-300 gm and 20 females weighing 200-250 gm with average age 4-6 months were used.The animals were divided into four equal groups; the first one was orally gavaged with saline and kept as control, the second group was orally gavaged with L-arginine (50 mg / kg B.wt) dissolved in saline and given daily for eight weeks, similarly the thi1- Service of both treated and control males with females in estrus was carried out, where at the 20th day of pregnancy the pregnant females were sacrificed and the number of corpora lutea on each ovary was counted as well as the number of implantation an2-Semen samples were collected from cauda epididymis for detecting the sperm motility, concentration and abnormalities.3- Sera were used for testosterone analysis and nitrite determination from both treated and control rats.4- Further more histopathological examination of testes and seminal vesicle was performed.The obtained results revealed the followings:1- Morphoteratologic effect of L-arginine (50 / 100 mg/kg B.wt) and L-NAME (25 mg/kg B.wt) on pregnant female rats serviced with treated male rats.a-The second group (females serviced with males orally gavaged l-arginine (50 mg/kg B.wt) showed no change in the number of both early and late resorption sites, the number of implantation sites and the number of live and dead feati, a significant increasb- The third group (females serviced with males orally gavaged l-arginine (100 mg/kg B.wt) showed a significant increase in the number of both early and late resorption sites, a significant reduction in the number of implantation sites, a significant reduc- The fourth one (females serviced with males orally gavaged L-NAME (25 mg/kg B.wt) showed a significant increase in the number of both early and late resorption sites, a significant reduction in the number of implantation sites, a significant reduction 2-Semen characteristics:A- Sperm motility:There was significant increase in sperm motility percentage in the second group from (73.2 ± 1.31%) in the control group to (92.5 ± 2.23 %). while the third and fourth group showed a significant reduction in sperm motility as compared to control (37 ± 8.7B- Sperm cell concentration:There was a significant increase in sperm cell concentration in the second group from (29.68 ± 2.28 ml x125x104) in the control group to (57.44 ± 3.063 ml x125x104). While the third and fourth group showed a significant reduction in sperm cell concentratiC-Sperm abnormalities:The second group showed a significant reduction in the percentage of abnormal spermatozoa from (15.6 ± 1.166%) in the control group to (10.6 ± 0.40%).While the third and fourth group revealed a significant increase in sperm abnormalities (30.4 ± 1.63 and 26.8 ± 1.24 % respectively) as compared with control (15.6 ± 1.166%).2- Serum testosterone level:The level of testosterone hormone was significantly increased in the second group with a mean value (6.53 ± 1.17 ng / ml) as compared with the control one (2.53 ± 0.32 ng / ml).In the third group, testosterone showed a significant reduction with a mean value (0.424 ± 0.07 ng / ml) comparing to control one (2.53 ± 0.32 ng / ml).The fourth group revealed no change in testosterone level.3-Serum nitrite level:The clinical finding showed that nitrite level showed no change in the second group, while the third one revealed a significant increase with a mean value (42.61 ± 4.68 ng / ml) as compared to control one (17.45 ± 0.45 ng / ml).The fourth group showed a significant reduction in nitrite level with a mean value (9.70 ± 0.30 ng / ml) comparing to control one (17.45 ± 0.45 ng / ml).4- Histopathological examination:The control group revealed normal histological picture of testis and seminal vesicle, while in the second group; the testis showed increased spermatogenesis with numerous sperms in the lumen of the seminiferous tubules.The seminal vesicle showed hyperplasia of the epithelial lining with increased activity, beside the presence of excessive secretion in the lumen- The third and fourth one showed degeneration and necrosis of the spermatogenic cells little number of spermatozoa in the testis, while the seminal vesicle showed atrophy of the epithelial lining, beside the presence of few Secretions in the lumen.Experiment IIFive healthy adult male rats were selected to study the effect of different concentrations of l-arginine on acrosome reaction. Sperm suspension was incubated in Tyrode, s albumin lactate pyruvate ( TALP) medium for 1 hour, then the percentage of acrosome The obtained results revealed the followings:After one hour of incubation l-arginine (5; 10; 20 m mol) produced a significant increase in the percentage of acrosome reacted spermatozoa, while at high concentration (40; 50 m mol) it resulted in a significant reduction in the percentage of acrosome re