Oxytocin immunoreactivity was mapped in the brain of adult guinea pigs. Emphasis was laid on cells and pathways other than those directed toward the neurohypophysis. Immunoreactive cell bodies were only detected in the hypothalamus and preoptic region. Compared to the rat, several differences were noted: there was no equivalent in the guinea pig, of the parvocellular neurones in the lateral and posterior parts of the paraventricular nucleus which are at the origin of caudally directed oxytocinergic pathways in the rat. There was also no equivalent to the rat anterior commissural nucleus. In contrast to the rat, numerous oxytocin-immunoreactive cell bodies were detected ventrally to the optic pathways, in retrochiasmatic and subchiasmatic areas and in the median preoptic nucleus. The close proximity of many oxytocin-containing cell bodies and dendrites to ventricular and subarachnoid spaces suggests that oxytocin could reach some of its targets following release into cerebrospinal fluid. Immunoreactive axons and presumptive axon terminals were found in the subfornical organ, in the dorsal motor nucleus of the vagus nerve and in the laterodorsal tegmental nucleus. Labelling was present, but weaker, in the following other areas: the dorsal parabrachial nucleus, the central nucleus of the amygdala and the nucleus of the solitary tract.