The firing patterns of visual cortical neurons were studied by intracellular recording in in vitro guinea pig brain slices. On depolarization 57% of the cells exhibited tonic firing of action potentials while the remaining cells (43%) had a phasic component in their response. Phasic cells exhibited a large diversity in their burst characteristics as well as in the burst dependence on the membrane potential. Ionic conductances underlying burst generation appeared to be also diverse, thus bursting neurons in the visual cortex cannot be grouped in a single, homogeneous population.