Intracellular recording was used in vitro to analyze electrophysiological properties of neurons in myenteric ganglia of guinea pig distal colon. The neurons were classified into six types based on their electrical behavior. Type 1 colonic neurons discharged action potentials throughout depolarizing current pulses and were otherwise similar to S/type 1 neurons found in the guinea pig small bowel. The second type had passive and active electrical properties similar to those of AH/type 2 myenteric neurons of the small intestine. These cells discharged only a single spike at the onset of depolarizing current pulses, and the spikes were followed by long-lasting hyperpolarizing afterpotentials. Excitability of the type 2 neurons was enhanced in the presence of elevated Mg2+ and reduced Ca2+, and the spikes were unaffected by tetrodotoxin. Type 3 colonic neurons showed fast synaptic potentials but did not generate action potentials. The majority of neurons were referred to as type 2 colonic neurons. Type 4 neurons discharged single action potentials only at the onset of depolarizing current pulses, and the spikes were not followed by prolonged hyperpolarizing afterpotentials. Unlike type 2 neurons, excitability remained unchanged in the presence of reduced extracellular Ca2+ and elevated Mg2+. Action potentials of type 4 neurons were suppressed or abolished by tetrodotoxin. A group of spontaneously active neurons was classified as type 5 colonic neurons. Type 6 cells were inexcitable and assumed to be glial cells.