The current study was carried out to evaluate the toxic effect of thallium on fertility of male albino rats as well as possible inhibition of adverse effects by using potassium as a prophylactic drug. The oral LD50 value of thallium was determined as 15.49 mg/kg B.wt.. In reproductive toxicity experiment, 90 male rats were divided into 6 groups the first group was a control group, the second group was given potassium chloride daily at a concentration level of 0.36 % in feed as a positive control group. The third and fourth groups (A, B) were administrated orally thallium at a concentration level 1/20 and 1/10 LD50 daily. The fifth and sixth groups (C, D) were intubated with 1/20 and  1/10 LD50 daily with the addition of potassium containing diet (0.36 %) for two months. The clinical symptoms observed were recorded. Five male rats were sacrificed at monthly intervals. The results revealed that thallium induced adverse effects in male reproduction in all thallium treated rats with or without potassium, whereas animals received thallium with potassium showed lesser effect. The effect was dose and time dependent and the most affection appeared in group (B which was given 1/10 LD50. These adverse effects were in the form of decrease in relative testes' weights; however, seminal vesicles and prostate glands weights were significantly increased in comparison to controls. Testicular sperm count, sperm motility, alive sperm percent- ages were Significantly reduced, meanwhile, sperm cell abnormalities increased. Fertility was significantly reduced in groups' dosed thallium with or without potassium in that the number of females impregnated by them was significantly reduced to record 0% in group (B). Abortion was observed in group (A); resorption, significant drop in number of implants/litters and alive