Cholecystokinin (CCK) is one of the most abundant peptides in the central nervous system (CNS). Radioligand-binding studies have identified and localized both central (CCK(B)-R) and peripheral (CCK(A)-R) receptor subtypes in the CNS. However, these studies have been limited by the relative specificities of agonists and antagonists for receptor subtypes and their inability to distinguish cell bodies from dendritic projection. Recently, we isolated and cloned the cDNAs for both CCK(A) and CCK(B) receptors. In the present study, (35)S-labeled cRNA antisense probes were synthesized for both receptors and in situ hybridization studies were performed in the rat brain, allowing a direct and independent comparison of the distinct distribution of expression of CCK(A) and CCK(B) receptor mRNAs in the rat brain for the first time. mRNAs for both CCK(A) and CCK(B) receptors were expressed mainly in the cortex, olfactory regions, hippocampal formation, septum, and interpeduncular nucleus. Only CCK(A) receptor mRNAs were expressed in some of the hypothalamic nuclei (paraventricular nucleus, arcuate nucleus, and medial preoptic area). In most of amygdaloid nuclei only CCK(B) receptor mRNAs were expressed. Although the presence of receptor mRNAs does not necessarily imply the presence of functional receptor proteins in the same location, this study showed the regional distribution of mRNAs for CCK(A) and CCK(B) receptors and provides important information about the distribution of CCK receptor subtypes in the CNS which will allow better resolution of their functional roles in the brain.