Organophosphorous pesticides are a healh problem worldwide, mostly for agroworkers, which are around 2600 millions of person in the year 2001. Little is known about male reproductive damage elicited by these chemicals. The acute effects of malathion (single intraperitoneal injection), 1/12 the LD50 to mice were analyzed at 1, 8, 16, 35 and 40 days after injection, in terms of spermatogenic cell proliferation and apoptosis and of Sertoli cell compromise as revealed by immunocytochemical detection of CK-18 (cytokeratin). The micronuclei test was done to assess for genotoxic activity of the agropesticide. The results showed decreased germ cell number followed by compensatory spermatogonial proliferation by 16 days (as shown by thymidine-H3 testicular uptake), increased apoptotic rate, mainly of spermatogonia, and preleptotene spermatocytes. Sertoli cell reactivity to CK-18, denoting alteration of them, probably due to germ cell damage, was seen in treated mice. Genotoxicity for somatic cells was demonstrated by the micronuclei test. Therefore, it is assumed that Sertoli as well as Leydig cells are affected together with spermatogenesis; recuperation of testicular morphology is seen by longer time intervals