A comparison of cholecystokinin (CCK) receptor binding to sections of rat, mouse and guinea pig brain has been performed using ¹²⁵I-Bolton Hunter CCK and the selective peripheral CCK receptor antagonist L-365,031. In both rat and mouse, ¹²⁵I-Bolton Hunter CCK binding in the region of the interpeduncular nucleus (IPN) was inhibited by L-365,031 indicating that these receptors resemble CCK receptors found in peripheral tissues. In the mouse especially, dense regions of peripheral CCK receptors were detected either side of the IPN. By contrast, in the guinea pig IPN no evidence of L-365,031-sensitive binding could be found. The present reports shows that in different species, regional variations in brain CCK receptor binding occur not only in the case of classical ‘brain’ receptors, but also for the more discretely localised ‘peripheral’ type CCK receptors.