Since the stress influences the biological processes, in the present study the effect of prenatal immobilization stress on the sexual activity of male rats was studied. Adult male and female Wistar rats (Pasteur Institute, Tehran) were copulated. After observing the vaginal plugs, pregnant rats were divided into control and two experimental groups. All groups were kept under the laboratory conditions, i.e. temperature adjusted at 22 ± 2 °C, a light and dark photoperiod of 13:11 hr and special food and water ad libitum. The pregnant control rats were under no stress, but experimental groups were put in the special adjustable restrainers from the beginning of the third week of pregnancy (14^th day) which is the development period of the nervous system and the sexual dimorphic nucleus (SDN). One of the experimental groups was immobilized in the restrainer for two hours daily (from 9 to 11 a.m.) and the other for three hours daily (from 8 to 11 a.m.) for one week. The offspring were stayed with their mothers in the same cage for 40 days. Then male young rats (control and experimental groups with 9 rats in each of them) were put in the cages with experienced adult male and receptive female for one day to become familiar with the sexual experience. Then control and experimental male rats were placed individually in cages, each containing a receptive female and their sexual activities were observed. Measurement of serum hormones showed a significant decrease in testosterone level in experimental group compared to the control group that can be attributed to a decrease in the size of SDN and changes in the histology of testis. Also the level of estradiol and progesterone was increased in the experimental group. In addition, the histological study of testis showed a decrease in the number and diameter of seminiferous tubules in experimental groups compared to control. In conclusion, it seems that prenatal immobilization stress affects testicular histology in male rats and also decreases the serum level of testosterone and sexual activities.