The adjustment of pineal melatonin and locomotor activity rhythms to 10:10-h light:dark (LD) or 14:14-h LD cycles was studied in male Wistar rats. Both lighting conditions were thought to be outside the limits of entrainment of the rest-activity rhythm in this species. We assumed that the rhythm of pineal melatonin synthesis might be more adaptable. As expected, the locomotor activity rhythm was not adjusted to the 10:10-h LD cycles. Under these conditions, a free-running component (25 h) became dominant. Under the 14:14-h LD cycles, however, an unexpected adaptation occurred within 10 days. The profiles of the pineal melatonin contents measured on days 5 and 30 under the 10:10-h LD and on day 7 under the 14:14-h LD schedule were in line with the estimated free-running oscillations, but the profile on day 21 under the 14:14-h LD schedule was not. This melatonin pattern fitted the LD-adjusted activity rhythm. Thus, the melatonin rhythm did not adapt better than the activity rhythm to the exotic LD cycles. Instead, parallel changes were found.