The distribution of substance P-like immunoreactivity (SP-LI) in the guinea pig brain has been studied by immunohistochemistry and the results compared with the distribution in similar regions in the rat brain. In both species, dense SP-LI staining was found in the median eminence, arcuate hypothalamic nucleus, substantia nigra, dorsal raphe and dorsal tegmental nuclei, nucleus of the solitary tract, substantia gelatinosa of the spinal trigeminal nucleus, and spinal cord. Less dense staining was found in the caudate putamen, globus pallidus, nucleus accumbens, habenula, hypothalamic areas, and central grey. SP-LI cell bodies were found in areas previously described for the rat brain including several hypothalamic areas, limbic areas, central grey, and dorsal raphe and solitary tract nuclei. The major difference between the two species was found in the cortex and hippocampus. The guinea pig cortex contained many more SP-LI cells and fibres, distributed in layers II-VI, than the rat cortex. The guinea pig hippocampus contained marked staining, particularly in the pyramidal cell layer of CA1-3 fields of Ammon's horn and in the granular layer of the dentate gyrus, and SP-LI cells in the hilus of the dentate gyrus, whereas rat hippocampus contained few cells and no regions of dense staining. It is concluded that because the guinea pig brain has an extensive distribution of SP-LI in the cortex and hippocampus it resembles the primate brain more closely than does the rat brain.