1 Reproductive toxicity and toxicokinetics of lindane during lactation were studied. For the reproductive toxicity study the dams were treated with a single dose of 6 mg/kg on day 9 or 14 of lactation, or with 1 mg/kg on days 9 to 14 of lactation. The male offspring were investigated at puberty and adulthood. For the toxicokinetic study, two groups of dams were treated with 6 mg/kg on day 9 or 14 of lactation. The concentration of lindane was measured in maternal plasma and milk, as well as in male offspring organs. 2 At adulthood, testicular weight and the number of sperm and spermatids were significantly reduced in all treated groups. 3 The testosterone level of the groups treated with 6 mg/ kg was significantly reduced to approximately 50% at puberty as well as in adulthood. In the group treated with 1 mg/kg, the testosterone level was in both age periods reduced, however, only at puberty was the reduction statistically significant. 4 The concentration oflindane in the testis was similar to that found in brain and was half the concentration found in the liver. 5 Histologically some areas of the testis exhibited distinct alterations ranging from small changes to a pronounced effect. 6 Exposure to lindane during lactation induces repro ductive hazards to male offspring rats which are detectable at adulthood.