The neuronal effects of somatostatin-14 (SS-14) and its influence on responses to acetylcholine (AcCho) were studied in vivo in the rat parietal cortex and dorsal hippocampus, using single-unit recording and microiontophoresis. SS-14 inhibited spontaneous firing of nearly all cells tested, while AcCho facilitated their firing. In contrast to its direct slowing effect, sustained iontophoretic application of SS-14 enhanced AcCho-induced excitations in 78% of all cells tested. This AcCho-enhancing effect of SS-14 was dose dependent. SS-14 did not enhance the responsiveness to pulses of the excitatory amino acid glutamate. Neurons tonically driven by iontophoretic currents of AcCho responded to concurrent pulses of SS-14 with an increase in firing. Thus, iontophoretic application of SS-14 can produce qualitatively different effects on the spontaneous activity of its target cells depending on the simultaneous effects of other chemical messengers. These condition-dependent interactions may explain the diverse neuronal effects of SS-14 reported in the literature.