SUMMARY An experiment was carried out to investigate the hormonal effect of gibberellic acid (GA3) on some physiological; reproductive parameters and hatchability performance of laying hens during winter and summer seasons of Egypt. A total of 120 hens (60 per each season) at the end of the production curve (62 wks of age) were randomly divided into three equal treatment groups that bi-weekly injected subcutaneously with 0; 200 or 400μg /kg of body weight of GA3 hormone for twelve weeks (whole experimental period). Egg number or weight and some egg quality parameters were of nearly similar values during winter or summer seasons. Most of these traits were slightly improved due to the injection by GA3, especially egg number and shell thickness that increased significantly (P<0.01) either with 400 or 200 μg of GA3 comparing with those of control group hens (21.07; 21.15 and 19.09 eggs, resp. and 0.36; 0.36 and 0.34 mm, resp.). Most of some haematological parameters were affected significantly due to season effects, except Hb and H/L ratio that showed nearly similar values during the two seasons. Hens during winter season had significantly (P<0.05 or 0.01) higher estimates of WBCs; PCV; heterocytes and lymphocytes and lower estimate of RBCs than those of hens in summer season. Values of WBCs; RBCs; PCV and lymphocytes were improved significantly (P<0.05 or 0.01) in hens injected by GA3, especially with the level of 400μg that caused a better improvement than that of the other one (200 μg). Values of Hb; heterocytes and H/L ratio didn’t differ significantly due to GA3 effects, however, Hb and heterocytes estimates tended to improve in hens treated with GA3. Hens in winter season recorded significantly (P<0.05 or 0.01) higher values of total protein; globulin and total lipids and lower value of glucose than those of hens in summer season. Also, albumin value was increased in winter season, but not significantly. GA3 increased all values of the blood components (total protein; albumin; globulin; glucose and total lipids). However, the increasing was significant (P<0.01) only in total protein and albumin (4.78 and 2.57 g/dl, resp.) in hens injected by 400μg of GA3 compared with either those injected by 200μg (4.44 and 2.24 g/dl, resp.) or control group (4.24 and 2.14 g/dl, resp.). Fertility and hatchability of total egg set or of fertile eggs percentages did not show any significant deference due to season effects, however, winter season had slightly higher values of these parameters than those of summer ones. On the other hand, chick weight or chick weight percentage were significantly higher (P<0.05) in winter season than those of summer ones (38.54 and 65.67 vs. 37.71 and 64.70, resp.). Hatchability of total egg set or of fertile eggs percentages and chick weight or chick weight percentage were tended to be slightly higher for hens injected with 400μg of GA3 than those of hens injected with 200 μg or those of control group ones. Fertility percentage was affected significantly (P<0.05) by GA3 treatment, where hens that injected by 400μg of GA3 showed the highest value (86.67 %) with 29 % more than that of control group (66.67%). Interaction effects between season and GA3 on studied traits were insignificant. In conclusion, most of haematobiochemical traits of blood and chick weight were improved during winter seasons. In addition, GA3 has a positive effect on productive performance and fertility of laying hens.