AbstractThis study describes the concentrations of melatonin in plasma samples taken from marmoset monkeys (Callithrix jacchus) every 4 h over three 24‐h periods. A circadian pattern of secretion was apparent, with higher levels recorded at night (20.00–08.00 h) than during the day (08.00–20.00 h) and a peak concentration at 20.00 h. There was a significant difference in the mean day and night concentrations (32.5 ± 4.5 pg/ml versus 49.0 ± 6.9 pg/ml, respectively) with individual concentrations ranging between<10–60 pg/ml in the day and 15–200 pg/ml at night. Circadian plasma melatonin concentrations were similar over the three 24‐h periods, in male (n = 3) and female (n = 3) monkeys, and in dominant (cyclic, n = 5) and subordinate (acyclic, n = 4) females. The results show a less pronounced circadian profile in the marmoset than is seen in the human but a similar profile to that in the seasonally breeding