Laboratory experiments were conducted to evaluate the teratogenic potential and developmental toxicity of  cypermethrin and diazinon pesticides in Japanese quail chick-embryos (COTURNIX JAPONICA). The effects of sublethal dosages (1/10, 1/50 and 1/100 LD₅₀) of tested pesticides on egg-weight and percent loss of egg weight during incubation were studied. It was quite clear that all treatments including the control showed positive increases in percentages loss of egg-weights throughout the incubation period. The effect of tested pesticides on egg-weight loss is not dose-dependent. On the other hand, cypermethrin, for instance, caused dose-dependent reductions in chick embryo weight representing 18.45, 35.56 and 49.87 % as compared to the control after being injected by 0.00075, 0.0015, and 0.0075 mg/egg, respectively. The same trend of results occurred with diazinon insecticide. The corresponding malformathion percentages were 20 and 38 % for cypermethrin and diazinon, respectively. In general the most pronounced malformations were body weight, wry neck, body size, growth retardation, abdominal hernia deformed legs and toes. The highest malformation was observed in diazinon-treatment followed by cypermethrin at the same concentration. The specific activity as well as the percent inhibition of acetylcholinesterase after 6, 9, 12 and 18 days of application are presented. It is quite clear that, cypermethrin and diazinon inhibit the activity of AChE by different ratios.