Cellular localization of platelet-activating factor (PAF) receptor in the rat brain was determined by (1) in situhybridization, (2) Northern blot analysis in primary cell cultures of neurons, microglia, astrocytes, and fibroblasts, and (3) Ca²⁺ imaging in hippocampal culture. In situ hybridization revealed that the PAF receptor mRNA is expressed intensely in microglia and moderately in neurons. Northern blot analysis revealed that PAF receptor mRNA is the most abundant in microglia. In primary hippocampal cultures, PAF elevated intracellular Ca²⁺ concentration in microglia and also in neurons, but to a lesser extent. These results suggest predominant presence of PAF receptor in microglia. Cultured microglia also expressed cPLA₂ mRNA the most intensely. PAF-activated microglia released arachidonic acid in a Ca²⁺-dependent manner and without conversion to its derivatives. We propose that microglia as well as neurons contribute to PAF-related events in the CNS by releasing arachidonic acid.