Protein kinase C (PKC) is involved in neural plasticity. The phosphorylation of the myristoylated alanine-rich protein kinase C substrate (MARCKS) in the left intermediate and medial hyperstriatum ventrale (IMHV) of the chick brain has been shown previously to correlate significantly with the strength of learning in filial imprinting. The distribution of PKC alpha, beta I, beta II and PKC gamma in the brain of 1-day-old dark-reared chicks was determined immunocytochemically, using the monoclonal antibodies MC5 and 36G9, raised against purified PKC alpha beta and PKC gamma, respectively. PKC gamma-stained cells were distributed widely in the telencephalon, including all hyperstriatal structures (including the IMHV), the hippocampus, neostriatum, ectostriatum and archistriatum. There were fewer stained cells in the septum and the least cellular staining was in the paleostriatum primitivum. Fluorescent double-labelling with neuron-specific enolase (NSE) and with the glial calcium-binding protein S100 suggested that PKC gamma immunoreactivity was present in neurones but not in glia. The distribution of PKC alpha beta-stained cells was more limited, with staining in the archistriatum, hippocampus and septum but not in the hyperstriatum. However, there was PKC alpha beta-staining of some fibres in the IMHV (but little elsewhere in the hyperstriatum ventrale), in the neostriatum, paleostriatal complex and the lobus parolfactorius. Double-labelling with NSE and S100 revealed PKC alpha beta/S100-positive glial cells present in the paleostriatal region only. There was some PKC alpha beta-staining of putative neurones in the hippocampus, septum and archistriatum. The differential distribution of PKC isoenzymes suggests that in the IMHV some axonal inputs contain PKC alpha beta whereas some postsynaptic cells contain the gamma form of PKC.