Plasma atrial natriuretic peptide (ANP), plasma renin activity (PRA), aldosterone (ALD) and vasopressin (VP) were assessed in six heart transplant patients (HTP) and ten healthy subjects under bed rest conditions and 60 and 120 minutes after head-out water immersion (WI). Bed rest had no significant influence on these parameters. WI raised plasma volume (PV) to the same extent in both groups. This increase of PV was accompanied by significant suppression of PRA, ALD and VP and an increase of plasma ANP. In HTP basal plasma ANP was significantly elevated and the ANP response to central hypervolemia reduced. Significantly elevated VP plasma levels were also found in HTP. These endocrine abnormalities in HTP seem to be caused by latent failure of the transplanted heart. No direct correlation was found between plasma ANP and PRA, ALD and VP under basal conditions and after WI in either HTP or normals.