1. The effect of urotensin II (U-II) on spontaneous transmitter release was examined in the frog to see if the biological activity of this vasoactive peptide extended to neural tissues. 2. In normal Ringer solution, frog and human U-II (fU-II and hU-II, respectively) caused concentration-dependent, reversible increases in miniature endplate potential (MEPP) frequency, with hU-II about 22 times more potent than fU-II. hU-II caused a dose-dependent increase in MEPP amplitude, whereas fU-II caused an increase, followed by a decrease with higher concentrations. 3. Increasing extracellular Ca(2+) three-fold had no effect on the MEPP frequency increase to 25 microM hU-II. Pretreatment with thapsigargin to deplete endoplasmic reticulum Ca(2+) caused a 61% reduction in the MEPP frequency increase to 25 microM hU-II. 4. Pretreatment with the phospholipase C inhibitor U-73122 caused a 93% reduction in the MEPP frequency increase to 25 microM hU-II and a 15% reduction in the increase in MEPP amplitude. Pretreating with antibodies against the inositol 1,4,5-trisphosphate (IP(3)) type 1 receptor using liposomal techniques reduced the MEPP frequency increase by 83% but had no effect on MEPP amplitude. 5. Pretreating with protein kinase C inhibitors (bisindolylmaleimide I and III) had no effect on the response to 25 microM hU-II, but pretreating with protein kinase A inhibitors (H-89 and KT5720) reduced the MEPP frequency increase by 88% and completely abolished the increase in MEPP amplitude. 6. Our results show that hU-II is a potent stimulator of spontaneous transmitter release in the frog and that the effect is mediated by IP(3) and cyclic AMP/protein kinase A.