The effect of pinealectomy on the circadian periodicity in plasma gonadotropins (LH and FSH), testosterone (T) and dihydrotestosterone (DHT) was investigated in adult male rats maintained under conditions of 12 h light/12 h dark (light on 06:00-18:00 h). Animals were killed at 3 hour intervals over 24 hours by decapitation. Plasma concentrations of LH, T and DTH (but not FSH) were found to exhibit significant circadian variations as determined by analysis of variance. No distinct LH or T nadir was recorded but peaks were observed at 09:00 h. Peak DHT values were recorded near the middle of the dark period. Pinealectomy resulted in an altered daily LH and T rhythmicity, and also in an abolished DHT rhythmicity. The fact that there was no significant increase in mean gonadotropin levels at 50 days after pinealectomy suggests that the pineal regulates the timing of gonadotropin secretion rather than the quantity. Since pinealectomy was found to abolish the circadian rhythm of DHT but not of T, we deduced that the pineal may also be responsible for the rhythmic conversion of T to DHT.