A single subcutaneous injection of 6 micrograms/100 g body wt. of neuromedin U-8 (NMU8) resulted in a transient increase in ACTH blood concentration (between 3 and 12 h) and a sustained (24 h) elevation of plasma corticosterone (B) concentration (PBC). After a 2-day subcutaneous administration of NMU8 (2 or 6 micrograms/100 g/day) blood ACTH level remained unchanged, but PBC was notably raised. Adrenal glands were atrophic, due to a marked loss of parenchymal cells, but in spite of this their basal in vitro output of B did not differ from that of control glands. These findings indicate that NMU8 stimulates pituitary-adrenocortical axis in rats. Furthermore, they suggest that, in addition to the enhancement in ACTH release, other mechanism(s) must be involved in the adrenocortical secretagogue effect of NMU8.