The major finding of the present study is that iontophoretically applied serotonin increased markedly (average 94.0%) the number of complex discharges within the majority (74%) of cerebellar Purkinje cells tested. Twenty-five percent of the cells showed an average 23% decrease in complex discharges, whereas 1% of the cells failed to respond. The effects of serotonin on complex activity were not related to any single effect of this amine on simple spike activity. It was apparent that the actions of serotonin on complex discharge activity were correlated with the initial simple-spike firing rate of the Purkinje cell and the predrug number of complex discharges. The other component of the complex discharge pattern analyzed in this study was the mean post-complex-discharge interval (MPCI). Purkinje cells evincing lower MPCI values were those in which serotonin increased the MPCI value preferentially, whereas cells in which serotonin depressed MPCI values exhibited higher predrug MPCI values. The serotonin antagonists methysergide and metergoline antagonized serotonin-induced enhancements in the numbers of complex discharges, whereas ketanserin failed to alter the response, suggesting a degree of receptor specificity. Comparisons between the present study and our previous work identifying a rate-dependent component to the actions of serotonin on simple spike activity are described.