Treatment of prepubertal female rats with delta-9-tetrahydrocannabinol (THC) delays first estrus and ovulation. The present study was conducted to determine if treatments initiated at different prepubertal ages would be equally effective in delaying sexual maturation and if changes in prepubertal hormone secretion accompanied the delay. Treatments with THC (10 mg/kg) or vehicle were initiated at 24, 27 or 30 days of age and continued until first estrus was detected by vaginal smears. In one experiment, serum luteinizing hormone (LH) and prolactin (PRL) were measured at 24, 27 or 30 days of age after different durations of treatment. First estrus was delayed (4.8 +/- 1.1 days) when THC treatment started at 27 days of age, but not when treatment started at 24 or 30 days of age. Serum LH was reduced after 1 or 4, but not 7, days of treatment; however, LH suppression was associated with pubertal delay only when it occurred at 27 to 30 days of age. Serum PRL, which increased with age, was suppressed after 1 day of THC treatment at 27 or 30 days of age, but only the former was associated with delayed puberty. Suppression was not evident at 24 days of age after 1 day of THC, or at other ages after treatment for longer than 1 day. These results define a narrow interval, encompassing the ages of 27 to 30 days, during which THC exposure resulted in pubertal delay. Such treatment reduced serum LH and PRL levels, but reductions also occurred with treatments that did not result in maturational delay. The relationship of hormone suppression to the delay thus remains unclear, although the timing of suppression might be a factor.