The effects of stimulating the modulatory proctolin-containing neurons (MPNs) on the pyloric rhythm of the stomatogastric ganglion of the crab, Cancer borealis, were compared with those produced by exogenously applied proctolin. The effects of both MPN stimulation and proctolin applications depend on the preceding physiological state of the preparation. Both treatments increase the pyloric cycle frequency dramatically in preparations that are slowly cycling, but they have little or no effect on pyloric cycle frequency in preparations that are already rapidly cycling. MPN and proctolin produce maximal pyloric cycle frequencies of about 1.2 Hz, although much faster pyloric frequencies are possible. MPN stimulation and proctolin applications affect the number of impulses fired in each burst by pyloric network neurons. MPN's excitatory actions are longer lasting when a preparation is active than when it is quiescent before stimulation. These data suggest that many of MPN's physiological actions result from its release of proctolin. Small unitary postsynaptic potentials evoked by MPN stimulation in the lateral pyloric neuron may indicate the presence of a second neurotransmitter in MPN.