Using the retrograde tracers horseradish peroxidase-wheatgerm agglutinin and gold particles conjugated to wheatgerm agglutinin apo-horseradish peroxidase in combination with an antiserum against glutaraldehyde-fixed GABA, it was examined whether the pontine nuclei of the cat receive projections from GABA-like immunoreactive neurons in the brainstem, diencephalon, or deep cerebellar nuclei, contributing to the GABA-like immunoreactive fibre plexus previously demonstrated in the pontine nuclei [Brodal et al. (1988) Neuroscience 25, 27-45]. Following tracer injections that covered both the pontine nuclei and the reticular tegmental nucleus in two cats, it was found that 125 out of 1166 (10.7%) and 29 out of 294 (9.9%) retrogradely labelled neurons in the cerebellar nuclei were GABA-like immunoreactive. In the same two experiments only six out of 2029 (0.3%) and 10 out of 1398 (0.7%) retrogradely labelled neurons in the brainstem and diencephalon were GABA-like immunoreactive. Among the regions in the brainstem and diencephalon known to project to the pontine nuclei, double-labelled cells were seen in the reticular formation, the periaqueductal gray, and the nucleus praepositus hypoglossi, but not in the zona incerta or the anterior pretectal nucleus, regions that have been shown to contain glutamate decarboxylase-like immunoreactive neurons projecting to the pontine nuclei in the rat [Border et al. (1986) Brain Res. Bull. 17, 169-179]. In order to test whether this is due to species differences, the same experimental approach was used in the rat, and it was found that 54 out of 3249 (1.7%) retrogradely labelled neurons in the brainstem and diencephalon were double-labelled. Notably, in the zona incerta 2% of the retrogradely labelled cells were also GABA-like immunoreactive, and in the reticular formation there was a higher proportion of double-labelled cells than was found in the cat. Additional sources were identified, that may contribute to the GABA-like immunoreactive fibre plexus in the pontine nuclei of the rat. This, in conjunction with the previous finding that the pontine nuclei of the rat contain only very few putative GABAergic neurons [Border and Mihailoff (1985) Expl Brain Res. 59, 600-614; Brodal et al. (1988) Neuroscience 25, 27-45], lead to the suggestion that the GABA-like immunoreactive fibre plexus in the pontine nuclei of the rat is predominantly of extrinsic origin, possibly representing a mosaic of the terminal fields of several subcorticopontine projections.(ABSTRACT TRUNCATED AT 400 WORDS)