Two novel and sensitive methods for quantitative determination of doxycycline hyclate and oxytetracycline HCl were developed. The first method (method A) is based on the reducing character of the cited drugs which causes chemical reduction of silver ions to silver nanoparticles (Ag-NPs) in the presence of polyvinyl pyrrolidone (PVP) as a stabilising agent producing surface plasmon resonance which has absorption peaks at 424 and 428 nm for doxycycline hyclate and oxytetracycline HCl, respectively. The nanoparticles were characterised by UV-VIS spectrophotometry and transmission electron microscopy (TEM). The plasmon absorbance of the Ag-NPs was used for the quantitative spectrophotometric determination of the cited drugs. The second method (method B) is a conductometric method which is based on the reaction of the cited drugs with phosphotungstic acid to form ion associates in aqueous system. Validation of the proposed methods was carried out.