1. Ganglion cells of the myentric plexus of the guinea-pig small intestine were studied with intracellular recording methods. 2. Electrical stimulation of the interganglionic connectives elicited slow synaptic excitation (slow e.p.s.p.) that was associated with an increase in the input resistance of the cell. The slow e.p.s.p.s continued for several seconds after termination of stimulation, and they occurred only in neurones in which prolonged hyperpolarizing after-potentials followed an action potential. 3. Superfusion of the neurones with solutions containing either 1-5 mM-Mn2+ or 16 mM-Mg2+ and 1 mM-Ca2+ mimicked the slow e.p.s.p. The common characteristics of Mn2+, Mg2+ and the slow e.p.s.p. were: (a) depolarization of the membrane potential, (b) increased input resistance of the cell, (c) augmented excitability, (d) blockade of post-pike hyperpolarizing potentials and (e) reversal potential between -70 and -75 mV. 4. Analyses based on the 'constant field equation' indicated that the permeability ratios of K+ to other permeant ionic species were reduced when Ca2+ influx was blocked by Mn2+ or Mg2+. 5. The organic Ca antagonist D-600 did not affect the neurones. 6. The results suggest that slow synaptic modulation of excitability within the myenteric plexus involves a reduction of both resting GK and post-spike GK which is secondary to suppression of Ca2+ influx by the neurotransmitter for the slow e.p.s.p.