Pineal melatonin content at several times during the day and night was measured in 36 inbred strains of mice (Mus musculus) kept under LD 12:12 cycles. The results have indicated that only five inbred strains have pineal melatonin content, with higher levels during the night and lower levels during the day; the other 31 strains do not contain detectable melatonin in their pineal gland at any of times examined. The former group includes two commonly used strains (C3H/He and CBA/Ms) and three wild-derived strains (Mol-A, Mol-Nis, MOM). C3H and CBA mice showed a similar pattern of pineal melatonin rhythm with a peak at 2 hours before lights on. The peak levels were about 150 pg/gland in both strains. The rhythmic patterns of melatonin content in Mol-A, Mol-Nis, and MOM were slightly different from those in CBA and C3H. In the wild-derived strains, the peak of melatonin content did not occur at 2 hours before lights on but tended to occur at midnight. The peak levels were 67-91 pg/gland at the highest point in these strains.