THE ADJUSTMENT OF THE MELATONIN RHYTHM OF RATS BY 90-MIN DARK PULSES

The pineal melatonin patterns were measured in rats kept 1, 2, 3, 4, 6 , 8, 16, or 28 days under the lighting conditions with a dark period o f 90 min only (from 18:00 to 19:30 h). The samples were collected at 3 0-min intervals, and melatonin was measured by radioimmunoassay. Durin g the first and second dark pulse the melatonin levels were low, but t hereafter they were significantly higher than the usual daytime levels . The increase of the synthesis started within 15 min after the light offset. The results suggest a stable adjustment of the melatonin rhyth m by the dark pulses.