During chronic treatment of weanling female rats with daily injections (0.069 mmol/kg of gamma-hexachlorocyclohexane (lindane, gamma-HCH), the treatment induced a significant 20% increase in body weight after 110 days. Further investigation with 0, 5, 10, 20 and 40 mg/kg lindane confirmed a significant increase in average body weight gain at the two highest doses after 10 weeks of treatment. Significantly greater food consumption was observed and the Lee Index indicated that lindane treatment induced obesity. In addition to obesity, lindane caused a delay in vaginal opening, disrupted estrous cycling, reduced pituitary and uterine weight and elevated food consumption during proestrus. These responses suggest that, by inducing alterations in the reproductive function of the female rate and by interfering with hormonal regulation of energy balance, lindane may be antiestrogenic rather than estrogenic as previously proposed. (Copyright (c) 1988 VCH Publishers, Inc.)