Neuronal and glial cultures from the hypothalamic-brain stem areas of 1-day-old normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rat brains stained positively with atrial natriuretic peptide (ANP)-specific antibodies. The endogenous levels of the ANP immunoreactivity in WKY neuronal and glial cultures were 17.0 +/- 2.2 and 14.3 +/- 2.7 pg/mg, respectively. Comparable neuronal and glial cultures from SH rat brains contained a 48 to 70% decrease in the endogenous ANP immunoreactivity levels. Culture media from both brain cell types also contained ANP immunoreactivity, the levels of which are significantly higher than those found in the cells. However, similar to endogenous levels, the media levels of immunoreactive ANP in SH neuronal and glial cultures were significantly reduced compared with WKY brain cultures. These observations demonstrate that endogenous ANP-like immunoreactivity is found in neuronal and glial cells and is released into the media. The levels of peptide are reduced in cultures of SH compared with WKY cultures, suggesting a genetically controlled difference between the hypertensive and normotensive rat strains long before hypertension develops.