Testicular weight and morphology, serum gonadotropins, intratesticular levels of testosterone and ABP levels in testicular interstitial fluid were studied in adult rats at intervals of 1 to 70 days after a single oral dose of 650 mg/kg methoxy acetic acid. At 3 days, this treatment resulted in the selective loss or depletion of pachytene and later spermatocytes from seminiferous tubules at all stages other than VIII to XI of the spermatogenic cycle. At later times this lesion was expressed as an absence mainly of round (14 days) or elongated (21 days) spermatids from the majority of seminiferous tubules. Other than these changes, spermatogenesis did not appear to be affected by treatment and was qualitatively normal in all tubules at 70 days after treatment. As deduced from cell counts at 3 days posttreatment, the initial action of methoxy acetic acid was restricted to late zygotene spermatocytes (stage XII) and pachytene spermatocytes at all stages other than early- to mid-stage VII. Levels of FSH in serum and those of ABP in testicular interstitial fluid indicated that Sertoli cell function was altered in rats treated with methoxy acetic acid. Both were increased at 1 to 3 days posttreatment, returned to normal at 7 to 14 days but were increased again at 21 days before finally returning to control levels at 28 days. In contrast, the levels of testosterone in serum, isolated seminiferous tubules and testicular interstitial fluid were unaffected by treatment, as also were the serum levels of LH. The two periods of increase in FSH and ABP levels coincided with the times of greatest decrease (approximately 20%) in testicular weight, and may be related either to the type of germ cell missing from the affected tubules and/or to the stage of the cycle of the affected (or unaffected) tubules. These data suggest that chemicals such as methoxy acetic acid may prove useful in the study of paracrine interactions in vivo.