1. Current-voltage relationship studies have shown that both synaptic and dopamine (DA)-induced burst-firing inhibition of cell R 15 in Aplysia californica are due to a reduction of the depolarization-activated inward current which underlies bursting. Bath application of tyramine (TYR), β-phenethylamine (PEA), and D-amphetamine (AMP) produced a similar reduction of this inward current. Kinetics experiments suggest that these three agents do not share a common mechanism with DA. 2. High doses of TYR, PEA and AMP induced synaptic activity through the activation of an interneuron. One form of this activity was inhibition of long duration following excitation (ILD-E). 3. Dihydroergotamine (DHET) blocked the effect of DA and the loss of inward current produced by TYR, PEA and AMP. The ILD-E, however, was not blocked by DHET. This suggests that these agents induce the generation of an ILD-E that is mediated by a transmitter other than DA, and that two receptors mediating burst-firing inhibition are present on cell R 15: a DA-sensitive receptor, and a DHET-insensitive receptor. © 1982.