The effect of the main psychoactive component of marihuana, delta-9-tetrahydrocannabinol (THC) was investigated on the onset of puberty and on the reproductive function in female rats up to the seventy-fifth to eightieth day of life. The drug was administered i.p. at a dose of 1 microgram/kg/day between the twenty-second postnatal day and the day of vaginal opening (V.O.). The administration of THC caused a 2-day delay in V. O., and the number of ova on the day of first estrus was significantly lower in treated rats than in controls. No differences were observed in serum gonadotropin and prolactin (PRL) levels on the day of V. O. After puberty, alterations occurred in the neuroendocrine functions of animals receiving THC that persisted until adulthood: estrous cycles were irregular, the number of ova in animals killed 35-40 days after V. O. was reduced, and serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels were decreased (diminution of serum FSH content was less expressed). An increase in serum PRL concentration could be demonstrated only in animals killed on the day of estrus. From these results, it might be concluded that THC administered to prepubertal rats--even in a very low dose--causes long-term irreversible alterations in reproductive functions. The importance of the fight against drug abuse is emphasized.