In urethane-anesthetized rats single neuronal activity was recorded in the laterodorsal tegmental nucleus, which has a dense complement of cholinergic neurons. In many experiments neuronal activity was recorded also in the dorsal raphe and locus coeruleus for comparison. In each experiment two to five active sites were marked with pontamine sky blue, deposited from the recording electrode ionophoretically and located histologically. Most neurons of the laterodorsal tegmental nucleus responded antidromically to stimulation of either one or more of the following sites: medial frontal cortex, medial septal nucleus, lateral geniculate nucleus or superior colliculus. Conduction velocities of their axons, estimated from latencies of antidromic responses, were within the range of unmyelinated fibers. Most of them gave rise to broad spikes (positive stroke, greater than 1.0 msec; negative stroke, greater than 3.0 msec), resembling monoaminergic neurons. Others generated brief spikes (positive and negative strokes, less than 0.8 and less than 2.2 msec). The former are probably cholinergic. Some neurons in the laterodorsal tegmental nucleus had rather regular spontaneous firing at a low rate (0.2-5 Hz), while others were not spontaneously active. Their response to noxious stimulation (tail pinch) was different from neuron to neuron, but that most frequently encountered was limited to phasic excitation, in contrast to locus coeruleus neurons which showed phasic excitation followed by mild tonic excitation; and to dorsal raphe neurons likely to be suppressed. Upon repetition of tail pinch the response of laterodorsal tegmental nucleus neurons tended to become weak or disappear while a desynchronized pattern continued to appear in the EEG in each occasion. Thus, putative cholinergic neurons, which constitute a major group in the laterodorsal tegmental nucleus, have properties similar to those of monoaminergic neurons, except for the phasic and decrementing nature of their response to tail pinch. The cholinergic, noradrenergic, and serotonergic ascending projections seem likely to subserve different roles in regulating activity of the forebrain.