Simultaneous in vitro measurements of electrical and mechanical activities were performed, using suction electrodes and force transducers, respectively, on longitudinal and circular muscle layers of the pig proximal colon. In addition, circular muscle strips were studied with the sucrose gap technique. Spontaneous activity was present in both preparations. In the circular muscle, slow waves with superimposed spikes occurred at a variable frequency, accompanied by phasic contractions. Longitudinal muscle preparations showed a different behavior. Regular appearance of distinct slow waves as described for the circular muscle did not occur. Instead, periods of membrane potential oscillations at a frequency of 41 cycles/min and a duration of approximately 12 s were observed in this layer. Most oscillations had superimposed spikes, and each period of oscillations was associated with a contraction. Spontaneous activity in the circular layer was myogenic in nature but susceptible to innervation and stretch. In contrast, an excitatory stimulus (acetylcholine or stretch) was a prerequisite for activity in the longitudinal layer. Cholinomimetics increased and adrenergic agents decreased the frequency of the slow waves and spiking activity and frequency and force of contractions in the circular muscle. Cholinergic agents increased the activity in the longitudinal muscle into continuous electrical oscillations with spiking activity and concomitant tonic contractile activity, whereas adrenergic agents abolished electrical and mechanical activity. Spontaneous release of acetylcholine occurred, partly due to regenerative activity of myenteric cholinergic nerves. In addition, tonic activity in the noncholinergic nonadrenergic inhibitory neurons decreased circular muscle tone.