Despite the significant contribution of the periaqueductal gray (PAG) to an endogenous pain suppression system, little is known about its neurochemical organization. Previous pharmacological and physiological studies have indicated regional variations in the effectiveness with which the midbrain PAG can generate potent analgesia in response to either opiate microinjection or electrical stimulation. There is, however, no anatomical correlate of this regional variation. As a first step toward elucidating the neural circuitry underlying the PAG's contribution to endogenous pain suppression systems, we have mapped the distribution of leucine enkephalin (ENK)-like immunoreactivity in the cat PAG. Throughout the rostral-caudal extent of the PAG, ENK- containing neurons are clustered in discrete populations. ENK terminal field staining is somewhat more diffuse; however, there are several regions where terminal staining is consistently more intense. The distribution of ENK perikarya and terminals undergoes a ventral to dorsal shift from caudal to rostral PAG. Conceivably, the clustered distribution of ENK cells and terminals contributes to the differential effectiveness of various PAG regions in generating analgesia. The ventral-dorsal shift of ENK immunoreactivity may (1) correspond to a somatotopic organization within the PAG or (2) mirror the topographic relationship of the PAG's interactions with other components of the endogenous analgesia system. In addition, the changing pattern of ENK immunoreactivity may also reflect the involvement of the PAG and of endogenous opiates in systems other than those of pain control.