Bombesin (BM) and neuromedin-B (NMB) exert similar biological effects, acting via two functionally distinct BM-receptor subtypes. The present study aimed to investigate whether BM and NMB stimulate the proliferation of rat adrenocortical cells and to compare their mode of action. Adult female rats were treated with a single subcutaneous dose of 3 micrograms BM or NMB. Adrenocortical proliferative activity was assessed by the metaphase-arrest technique. BM administration resulted in a marked increase in the number of metaphases in zona glomerulosa (ZG) and zona fasciculata (ZF), and in the entire cortex. This increase appeared 24 h after injection in the ZG, and after 48 h in the ZF. NMB administration, on the other hand, caused a prompt increase in the number of metaphases in the ZG and entire cortex at 12 h, followed by a subsequent drop below the control level at 24 and 48 h of experiment. These findings indicate that BM and NMB enhance the proliferative activity of rat adrenocortical cells acting via different receptors or different mediators.