We compared the Purkinje cells of adult normal and weaver mutant (wv/wv) mice by iontophoretic and electrophysiological tests. Although weaver Purkinje cells fire spontaneously at a rate (38 Hz) similar to normal mouse neurons (40 Hz), several abnormalities of firing were seen: high frequency bursts of single (simple) spikes occurred in 5-10-sec episodes in 38% of weaver cells, compared to 8% in normal mice; spontaneous complex spikes (climbing fiber-like burst responses) occurred in several different forms in a given Purkinje cell. As in normal mice and rats, the spontaneous single spike activity is readily depressed by electrical stimulation of the locus coeruleus, the presumed source of a dense noradrenergic plexus in the weaver cerebellar cortex. In a preliminary experiment the adrenergic blocking agent, fluphenazine, antagonized the responses to locus coeruleus stimulation. Iontophoresis of norepinephrine (NE), GABA and serotonin (5-HT) also uniformly depressed Purkinje cell single spike activity in all normal and weaver mice; cyclic AMP depressed 55% of normal and 70% of weaver Purkinje cells. Glutamate was always excitatory. The only qualitative difference was seen with acetylcholine, which was mostly inhibitory in normal mouse, but increased the firing rate in 42% of weaver Purkinje neurons. Cyclic GMP was predominantly excitatory in both types. Thus, despite the absence of parallel fibers, weaver Purkinje neurons grossly resemble normal Purkinje cells electrophysiologically as well as morphologically. Since several sites of indirect presynaptic actions are eliminated in weaver, our results further substantiate the direct post-synaptic inhibitory nature of GABA, 5-HT and NE, and the noradrenergic pathway from locus coeruleus to Purkinje cells. Similarly, consistent inhibitory responses to cyclic AMP in the weaver support the previously hypothesized role of cyclic AMP in the post-synaptic inhibitory response to NE.