There is limited information about the development of the primate circadian system. In adult mammals, circadian oscillation is driven by a transcription-transduction feedback loop of the clock genes Bmal1-2, Per1-3, Clock, Cry1-2. We investigated in adult and fetal capuchin monkey 1) expression of Bmal1, Per2, Clock and Cry2 in SCN and adrenal gland, 2) oscillatory expression of Bmal1 and Per2 in SCN and adrenal gland in vivo and 3) whether, Bmal1 and Per2 show an oscillatory expression in adrenal gland in vitro. We obtained SCN and adrenal glands from 8 adult male capuchin monkeys sedated with ketamine (10 mg/kg) and euthanized with i.v. sodium thiopentone (100mg/kg) at 0200, 0800, 1400 (n=3) and 2000 hours (n=3) and from 7 late gestation fetuses delivered by histerotomies at 1400 (n=4) and 2000 hours (n=3), euthanized by injection of sodium thiopentone (100mg/kg) in the umbilical vein. mRNA levels were measured by RT-PCR using 18S-rRNA (18S)as housekeeping gene. Bmal1, Per2, Clock, Cry2 were measured in tissue obtained at necropsy and Bmal1 and Per2 in adrenal explants incubated and collected in sets, each 6 or 4 hours for 48-h from from 4 adults and 3 fetuses. Approval was obtained from the local Ethics Committee. Bmal1, Clock, Per2 and Cry2 were expressed in adult and fetal SCN and adrenal gland. Bmal1 expression was higher in the adult adrenal gland (56.1±15.8 ng relative to 300 bp DNA standard) than in the SCN (9.9±3.3 ng), P<0.05, unpaired Student′s t test. Per2 and 18S levels were similar. Bmal1/18S in the SCN was higher at 2000-0200 than at 0800-1400 (P=0.06, unpaired Student′s t test), whereas in the adrenal values at 1400 and 2000 were higher than the two values obtained at 0200-0800. In the fetus, the expression of 18S and clock genes in the SCN and adrenal gland was lower than in the respective adult tissues and we did not detect differences in mRNA levels of Bmal1 or Per2 between 1400 and 2000 hours. In culture, both adult and fetal adrenals showed gene oscillation. In adult adrenal cultures Bmal1 mRNA showed a peak at 1400-2000. In fetal adrenal cultures, Per2 mRNA level showed a peak at 2400 (P<0.05, ANOVA). Conclusion: Clock genes are expressed in adult and fetal capuchin monkey SCN and adrenal. We found oscillatory Bmal1 expression in adult SCN. The expression of clock genes in the adult and fetal adrenal gland and their oscillation in culture suggests that these glands are peripheral circadian oscillators.