The somatotopic organization of structures in the midbrain which mediate stimulation-produced analgesia (SPA) was studied by stimulating a series of neural loci using a movable electrode. The strength and distribution of analgesia at 7 fields on the body surface, as measured by the application of noxious pinch, was clearly related to the dorsoventral stimulation locus. SPA at dorsal sites was distributed at the ears and, to a lesser extent, at the forepaws. At progressively more ventral loci, maximal analgesia appeared successively at the forepaws, hindpaws and tail. This organization was maintained throughout the rostrocaudal extent of the midbrain. However, rostral stimulation sites tended to produce analgesia in smaller, more discrete fields than those produced by caudal stimulation sites. Further studies showed that the relationship between SPA and stimulation site is influenced by the electrical stimulation current level. Each site had an optimum current level, so that current intensities higher or lower than the optimum produced decreased analgesia. These results reveal a possible neural mechanism for somatotopic organization in hyperstimulation analgesia in which an intense somatic input at a body site produces analgesia in the same or adjacent segments.