Abstract--To identify central neurons participating in cardiovascular regulation in hypertension, we studied Fos staining, a marker for synaptically activated neurons, in adult male normotensive and hypertensive (HT) rats. At 1 and 4 weeks after induction of unilateral nephrectomy, renal wrap hypertension mean arterial pressure was 138±4 mm Hg (n=6) in 1-week HT rats and 159±6 mm Hg (n=6) in 4-week HT rats. Mean arterial pressure was 103±2 mm Hg (n=6) in sham-operated, normotensive rats. Mean arterial pressure was greater in both HT groups compared with normotensive rats, and the mean arterial pressure in 4-week HT rats was greater than that in 1-week HT rats. Rats were anesthetized and perfused, brains sectioned and processed using a Fos antibody, and the number of Fos immunoreactive neurons counted in sections through various brain regions. Hypertension of 1 or 4 weeks did not alter the number of Fos immunoreactive neurons in the area postrema, the supraoptic nucleus, and the median preoptic nucleus. The number of Fos immunoreactive neurons was increased after 1 and 4 weeks in the nucleus of the solitary tract, both the caudal and ventral lateral medulla, and the organum vasculosum of the lamina terminalis. In addition, after 4 weeks of HT, the number of Fos immunoreactive neurons was increased in the parabrachial nucleus and the paraventricular nucleus of the hypothalamus. The results indicate central regions active in acute and chronic HT rats and suggest certain areas that may be differentially activated depending on the duration of the hypertension.