The molluscan neuropeptides, small cardioactive peptides A and B (SCPA,B), are known to modulate the responses of many molluscan central and peripheral target cells (see review by Lloyd, 1986), but their full range of physiological actions remains unknown. External application of SCPB (1-10 microM) modified diverse ionic conductances in a set of giant identifiable neurons in the brain of the marine mollusk Hermissenda crassicornis. SCPB caused a transient depolarization and increased input resistance that enhanced or promoted cell firing. Under voltage-clamp, SCPB reduced a "background" residual current (IR), reduced early transient K+ current (IA), reduced a delayed K+ current (IK(V], and enhanced ICa, IBa, and a Ca2+-activated K+ current, IK(Ca). In tetraethylammonium chloride (TEA) saline, SCPB enhanced the amplitude and duration and reduced the threshold of evoked Ca and Ba spikes. Immunocytochemical staining techniques have localized an endogenous SCPB-like peptide in numerous somata and their neurites in the nervous system of Hermissenda (Longley and Longley, 1985; Watson and Willows, 1986). These data are consistent with a role for SCPB as a neurotransmitter/neurohormone modulator of neuronal excitability in Hermissenda. A neurotransmitter role for endogenous SCPs has been proposed for a synaptic pair of cultured neurons in the Aplysia buccal ganglion (Lloyd et al., 1986). SCPB has been implicated in the control of feeding motor output in Aplysia (Sossin et al., 1986) and Tritonia (Willows and Watson, 1986), and in the presynaptic facilitation of sensory neurons mediating the gill and siphon defensive withdrawal reflex in Aplysia (Abrams et al., 1984).