The endocrine effects of gossypol were studied in 26 men for 52 months before, during and after treatment. A further 34 subjects were studied after cessation of gossypol treatment. A control group of 60 age-matched volunteers were monitored for 1 year. No significant changes in testosterone levels were observed during the course of study. Serum LH levels were, however, significantly higher (P less than 0.001) during earlier phases of gossypol treatment and returned to normal after cessation of treatment except in 14 men with persistent disruption of spermatogenesis. In the latter, serum levels of LH and testosterone were significantly higher (P less than 0.001) than those found in normal subjects and significantly lower (P less than 0.05) than in those subjects in which spermatogenesis had recovered. Serum FSH levels did not rise during the first 8-9 months of treatment with gossypol, although azoospermia generally occurred within 76 days of commencing treatment. After 9 months of treatment, serum FSH levels rose gradually and remained significantly elevated (P less than 0.001) after cessation of treatment in both the azoospermic/oligospermic group and in the group in which spermatogenesis recovered. In the latter group the serum levels of FSH were correlated significantly with the sperm concentration.