Fifteen prepubertal male rats (age 30 days) were divided into three groups of five each: Group 1, hypothyroid (Tx)--rats were thyroidectomized at day 30; Group 2, T4 (L-thyroxine) replacement therapy (Tx+T4)--rats were thyroidectomized at day 30 and treated daily i.m. with T4 (6 micrograms/100 g body weight/day) for 30 days from day 31 to day 60 post-thyroidectomy (age 90 days); Group 3, control--rats were sham-operated and treated with vehicle. The rats from all groups were killed on day 61 post-thyroidectomy or post-sham operation (age 91 days). The serum levels of testosterone, T4 and T3 decreased in the Tx group (p < 0.001). In the Tx+T4 group the levels of T4 and T3 were restored to control values, whereas testosterone levels remained lower than in the control group. Hypothyroidism caused various changes in the levels of epididymal phospholipids and neutral lipids. These were restored differentially or were altered further in the Tx+T4 group. The number and forward motility of spermatozoa, recovered from the cauda epididymis, were decreased significantly (p < 0.01) in the Tx group and were not restored in the Tx+T4 group. This study shows that chronic hypothyroidism, induced at prepuberty and continued for 60 days, causes various changes in the lipid composition of the caput and cauda epididymis and lowers the quality and quantity of spermatozoa in the cauda epididymis. T4 therapy for 30 days, especially during the postpubertal period, did not restore the quality or quantity of spermatozoa and caused differential changes in the levels of epididymal lipids, depending upon the region.(ABSTRACT TRUNCATED AT 250 WORDS)