Specific, sensitive and direct radioimmunoassays have been used to determine the daily patterns of 5-methoxytryptophol (ML) and melatonin in the pineal glands of Syrian hamsters kept in different photoperiods: 8 h light: 16 h darkness (8L:16D), 14L:10D and 16L:8D. A rhythm in pineal ML was evident in animals in all the photoperiods, with high daytime levels (641 +/- 35 (S.E.M.) fmol/gland; n = 162) which dropped to 119 +/- 16 fmol/gland (n = 44) 7.1-7.5 h after lights out. The duration of low night-time ML levels was proportional to the length of the dark phase (1.2 h in 16L:8D, 5.4 h in 14L:10D and 8.4 h in 8L:16D). A marked daily rhythm in melatonin was also present in hamsters in the different photoperiods, with daytime levels of 323 +/- 34 fmol/gland (n = 129) and night-time peak concentrations of 3676 +/- 336 fmol/gland (n = 22). The duration of high nocturnal melatonin levels was dependent upon the length of the dark phase (4.1 h in 16L:8D, 4.5 h in 14L:10D and 12.5 h in 8L:16D). Linear regression analysis revealed a statistically significant inverse relationship between pineal ML and melatonin levels in 8L:16D (P less than 0.001), 14L:10D normal (P less than 0.05) and 14L:10D shifted (P less than 0.001) photoperiods. After advancing the lighting schedule by 10 h (14L:10D, lights off at 04.00 h), pineal ML and melatonin rhythms became entrained to the new lighting regimen. The daily rhythms in pineal ML and melatonin in the Syrian hamster thus depend on the prevailing photoperiod, a reciprocal relationship existing between pineal ML and melatonin concentrations.