In a systematic attempt to trace all projections from the brainstem and diencephalon to the pontine nuclei of the cat, implantations and injections of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) or Fluoro-Gold were placed in the pontine nuclei of 21 cats. In most of the cases there was no evidence of spread of tracer outside the pontine nuclei. Retrogradely labeled cells in the brainstem and diencephalon were carefully mapped and counted. The number labeled cells in the brainstem and diencephalon ranged from 24 in cases with very small implantations to 3,490 in cases with large injections in the pontine nuclei (counts from every fifth section). The labeled cells are located bilaterally with an ipsilateral preponderance. After large injections, 25-38% of the labeled cells were located in the brainstem reticular formation, 10-16% in the pretectal nuclei, 10-15% in the hypothalamus, 7-9% in the zona incerta, 3-9% in the fields of Forel, 4-5% in the nucleus locus coeruleus, 3-5% in the ventral lateral geniculate body, 2-4% in the superior colliculus, 3% in the periaqueductal gray, and 14-15% in other parts of the brainstem. Judging from cases with small tracer deposits entirely confined to the pontine nuclei, there appear to be two types of subcortical inputs: Projections from the reticular formation, the nucleus locus coeruleus, the periaqueductal gray, and the raphe nuclei are widespread, presumably reaching all parts of the pontine nuclei, while projections from a diversity of other sources are localized, reaching limited parts of the pontine nuclei only or predominantly.