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 of lindane 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 reproductive hazards to male offspring rats which are detectable at adulthood.