Using an antiserum against conjugated histamine we mapped the histaminergic somata and their fiber projection areas in carbodiimide-fixed guinea pig central nervous system. The neurons were large and they were found exclusively in the posterior hypothalamus, as in the rat, but in the guinea pig they were more numerous and distributed more widely in thin layer around the posterior mammillary nucleus, scattered between and within the medial mammillary nuclei, and in a dense cell cluster emerging from the caudal magnocellular nucleus and extending to the medial preoptic area. The density of histamine-immunopositive fibers was very high in the olfactory tubercle, diagonal band of Broca, nucleus accumbens, medial and cortical amygdaloid nuclei, periventricular and lateral basal hypothalamus, paraventricular thalamus, and in a region from the medial central gray to the locus coeruleus and the parabrachial nucleus. Dense fiber networks were found in the piriform and entorhinal cortex, septum, dentate gyrus, and subiculum, in most parts of amygdala, and in many areas of the hypothalamus, thalamus, substantia nigra, raphe nuclei, inferior olivary, solitary tract and medial vestibular nuclei, and neurohypophysis. Medium fiber density was observed in the internal layers of the olfactory bulb, anterior olfactory nuclei, neocortex, zone CA1 of hippocampus, and many midbrain and hindbrain regions. Low density was present in the outer layers of the olfactory bulb, other parts of hippocampus, the globus pallidus, most of the caudatus-putamen, the cerebellar cortex, and the dorsal horn of the spinal cord. The retina and most of the myelinated white matter had single or no histaminergic fibers. It may be concluded from the results that most fibers seem to follow a ventromedial route to the forebrain, reaching the amygdala ventral to the medial forebrain bundle, the hippocampus via subiculum, and the hindbrain structures via the medial central gray. As compared to the rat, the fiber projections in the guinea pig brain were denser, particularly in the hippocampus, thalamus, pons-medulla, and neurohypophysis. The fiber densities in various regions of the guinea pig brain are compared to histamine receptor densities and the possible functions of histamine are discussed.