The present study is undertaken to evaluate the effects of hypothyroidism induced by carbimazol in male rat reproductive system. Rats were treated with carbimazol (80 and 160 mg/kg body weight) daily for two, four, six, and eight weeks, while control rats were given distilled water orally. The results of these experiments are: 1-Treatment with carbimazol produced increase in body weight of treated animals. This increase was not statistically significant even at high concentrations. 2-Decrease in testicular weight  was observed during all administration points. This decrease was aggravated proportional to duration and dose (p≤0.001). 3-Results of most intervals showed significant decrease in epididymal-head weight, while no significant decrease was observed in epididymal-tail weight. 4-Carbimazol increased seminal vesicle weight, particularly after six (P≤0.05) and eight weeks for both doses. 5-Prostates were responsive to reduce thyroid hormones by increasing its weight in a dosedependent manner.