While the general toxicity of the benzimidazole pesticides for mammals is low, one of these compounds, Carbendazim, causes degeneration of testicular tissue and decreases spermatogenic activity at doses well below the LD50 value. An earlier study showed that treatment with 400 mg/kg/d MBC resulted in severe seminiferous tubular atrophy and infertility. Since spermatogenesis is an androgen dependent process, the study characterizes the effects of MBC (0-400 mg/kg/d) on the endocrine function of the rat testes. The 400/kg dose resulted in increased concentration of both testosterone and ABP in the IF and SNF and elevated serum ABP, with no change in serum testosterone. It is concluded that SNF testosterone may be a result of two factors, increased IF testosterone concentrations and decreased testosterone outflow from the testis to the general circulation. Also, increased ABP in the IF may reflect a change in the relative secretion of ABP into the IF and the seminiferous tubules. (Copyright (c) 1989 American Academy Press, Inc.)