We have found changes in the morphology of photoreceptor somata from the mollusc Hermissenda that are produced by application of 12,13-phorbol dibutyrate (PDBU), an activator of PKC, in combination with elevated intracellular Ca2+ levels. The changes in morphology were expressed as rapid and repetitive outgrowths and additionally as more general changes in shape of the soma. Application of 4 alpha-PMA, a phorbol ester which does not activate PKC, did not produce these changes. The functional integrity of the photoreceptors in these dissociated eye preparations was maintained throughout the period of incubation with PDBU according to standard electrophysiological criteria. It has previously been shown that classical conditioning produced a reduction of dendritic volume in the type B photoreceptor of Hermissenda, a specific locus for associative memory storage. These changes in dendritic morphology were correlated with increased resistance across the cell membrane caused by learning-induced reductions of outward somatic K+ currents. Such conditioning-specific reductions of somatic K+ currents appear to depend on the phosphorylation of a 20-kDa G-protein (CP20) mediated by the Ca2+ and phospholipid-dependent kinase, protein kinase C (PKC). Thus PKC activity may be important in structural changes of the synaptic region of specific neurons involved in associative memory. The results of the present study suggest that the effects of PKC activation may also include structural changes in the soma of these same neurons.