Quantitative evaluation of the different varieties of germ cells at stage VII of the seminiferous epithelium cycle, namely type-A spermatogonia (ASg), preleptotene spermatocytes (pLSc), mid-pachytene spermatocytes (mPSc) and step 7 spermatids (7Sd), along with radioimmunoassay of plasma gonadotrophins (FSH and LH), testosterone and testicular testosterone were performed in Wistar rats following treatment with aldrin (polycyclic chlorinated hydrocarbon insecticide) for approximately one (13 days) or two cycles (26 days) of the seminiferous epithelium. Extensive degeneration of all varieties of germ cells at stage VIII, reduction in the sperm count and significant reductions in plasma concentrations of LH and testosterone were observed following aldrin treatment. The reduction in plasma concentrations of FSH was statistically significant only after treatment for two cycles. The inhibitory effect of aldrin on plasma gonadotrophins, testosterone levels, testicular testosterone content and numbers of 7Sd and ASg was maximum after treatment for two cycles. Administration of human chorionic gonadotrophin along with aldrin treatment for two cycles partially prevented the degeneration of germ cells and enhanced testosterone production. The results indicate that aldrin may have a direct inhibitory influence on gonadotrophin release, but the possibility of a direct action of the insecticide at the level of the testes is also discussed.