Effects of a single dose of naloxone and of D-Met2-Pro5-enkephalinamide on the DNA synthesis in the forebrain, hypothalamus and cerebellum of 11 day old female rats were studied. As an index of DNA synthesis the rate of incorporation of 3H-thymidine into DNA was measured 30 min after a sc. injection of 40 muCi/100 g b.w.. A time dependent effect of naloxone administration on cerebral DNA synthesis was observed. In the forebrain at 1 and 3 hrs after naloxone injection an increased rate of 3H-thymidine incorporation into DNA was found followed by a marked decrease at 9 and 12 hrs. The effect in the hypothalamus was similar but the initial increase at 1 hr was absent. On cerebellar DNA synthesis naloxone had no effect. The administration of D-Met2-Pro5-enkephalinamide resulted in a marked reduction in the labelling of cerebral and hypothalamic DNA between 1 to 12 hrs. Except a decrease at 1 hr no effect was found in the cerebellum.