Photoperiodic regulation of serotonin N-acetyltransferase activity in the pineal gland of the chicken.

1. In continuous light, serotonin N-acetyltransferase activity in the pineal gland of 16-day-old chicks was reduced to about a half of the usual nocturnal level only in the first day, but no suppressive effect by the light was observed throughout the following 2 days. 2. In continuous darkness, circadian N-acetyltransferase rhythm persisted for three consecutive days. 3. In continuous light, the phase of N-acetyltransferase rhythm in chick pineal delayed remarkably for three consecutive days, while the phase delayed in a lesser degree in continuous darkness. 4. The rhythm of N-acetyltransferase activity in the pineal gland of 13-day-old chicks was entrainable to a shorter (8L: 8D) and a longer (18L: 18D) photoperiod as in a 12L: 12D photoperiod, but a shorter photoperiod such as a 3L: 3D photoperiod failed to change the pre-established 12L: 12D rhythm.