The clinical trials of gossypol at the dosage of 15 mg and 22.5 mg per day were carried out in the city and the rural area respectively. The results of cytological, cytochemical as well as ultrastructural studies of the semen obtained before and during gossypol administration were as follows:1. It was shown that a decrease in sperm count and the occurrence of abnormal sperm in semen was detectable 4 weeks after gossypol administration. By the 8th week the sperm count being reduced markedly in semen, while exfoliated spermatogenic cells (including mainly spermatids and spermatocytes) increased concomitantly, the date indicated that the site of gossypol action was at the spermiogenic spermatids as well as the mid-and late stage spermatocytes.2. Decrease in the activity of SDH in the middle piece of the sperm and damage to the spiral mitochondrial sheath including derangement, vacuolation, and loss of mitochndrial cristae were observed following gossypol treatment. Such changes suggested that gossypol might suppress the energy matoblism of sperm. The effect of gosspol on the PLE of acrosome system appeared to be secondary.3. The ultrastructural changes of sperm nuclear chrmatin after gosspol treament suggested the possibility that gosspol might exert effect on the matoblism of the nuclic acid.