1. The mechanical and electrical activity of the smooth muscle of the distal colon of the guinea-pig has been recorded in experiments designed to determine the nature of the nerve-mediated excitation of the muscle.2. The spontaneous contractions of the colon, normally observed in vitro, were similarly antagonized by hyoscine or tetrodotoxin. However, neither drug caused complete cessation of the spontaneous activity.3. The contractile responses of the colon to repetitive stimulation of intramural nerve fibres were of two types, a primary contraction which occurred within 1 sec of the beginning of stimulation and a secondary contraction which occurred after stimulation. The primary contraction was blocked by hyoscine, usually revealing an inhibitory response to stimulation, but the secondary contraction persisted. Similar primary contractions were observed in response to stimulation of the pelvic and sometimes of the periarterial nerves.4. An initial relaxation during, and a secondary contraction after, stimulation were sometimes obtained when the periarterial nerves were stimulated. Propranolol completely blocked both the initial relaxation and the secondary contraction in response to sympathetic stimulation but did not affect either the relaxation or the secondary contraction in response to transmural stimulation.5. The direct effects of noradrenaline and adenosine triphosphate on the mechanical activity of the colon were studied. Both drugs caused a relaxation of the colon. Washout of either drug after a short exposure was followed by a period of increased activity. Especially in preparations of low tone, the initial relaxation and the secondary contraction in response to transmural stimulation could be mimicked by the application and washout of either noradrenaline or adenosine triphosphate.6. Two types of potential change were evoked in the muscle cells of the colon, either separately or in combination, when the intramural nerve fibres were stimulated: excitatory junction potentials (EJPs), which were blocked by hyoscine, and inhibitory junction potentials (IJPs). When the membrane potential recovered following an IJP, it was common to observe action potentials in muscle cells that were initially quiescent. A similar secondary firing of action potentials was initiated by hyperpolarizing the muscle cells with anodal current pulses in the presence of tetrodotoxin in sufficient concentration to block nerve-mediated responses.7. The results reported in this paper lead to the conclusion that the muscle cells of the distal colon of the guinea-pig are influenced by three sets of nerves: cholinergic excitatory, adrenergic inhibitory and intrinsic inhibitory fibres releasing a non-adrenergic transmitter substance. The secondary non-cholinergic excitation arises from a non-specific reaction of the muscle cells to a preceding inhibition.