The diurnal time course of pineal melatonin synthesis was analyzed simultaneously in the arginine vasopressin (AVP)-deficient Brattleboro rat (BB), the Long-Evans (LE) and Sprague-Dawley (SD) rat by means of radioenzymatic determination of the rate-limiting enzyme serotonin-N-acetyltransferase (NAT) and the melatonin content over a period of 24 h. While all 3 strains displayed a distinct day-night rhythm of melatonin synthesis (low day-time, high night-time values), BB rats generally exhibited lower NAT values as compared to LE and SD rats, though reaching a significant difference at 02.99 h only. Twenty-four-hour melatonin content was characterized by distinct nocturnal maxima in LE and SD rats, while BB rats showed a plateau-like nocturnal time course. Electrophysiological and pharmacological findings in SD rats point to an inhibitory influence of AVP upon pineal melatonin synthesis. The lack of AVP obviously does not result in disinhibition of pineal melatonin synthesis but rather in a different time course of pineal melatonin content. This might either be due to strain differences or to yet unknown compensatory mechanisms in BB rats.