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.