Cardiac beta-adrenoceptors were studied in membrane fractions from spontaneously hypertensive rats (SHR) and rats with two-kidney, one clip hypertension (2K, 1C HT), using radioligand binding method. beta-Adrenoceptor concentration measured by [3H]-dihydroalprenolol (DHA) binding was significantly lower in cardiac membranes from two months old SHR than those from Wistar-Kyoto rats (WKY) (38.2 +/- 2.6 vs 45.1 +/- 1.8 fmol/mg protein, means +/- SEM, p less than 0.05). Cardiac membranes from 2K, 1C HT rats had also a lower concentration of beta-adrenoceptors than those from the sham-operated control rats at a week after operation (30.9 +/- 2.2 vs 47.8 +/- 1.6 fmol/mg protein, p less than 0.01). But receptor affinity remained unchanged. These reduced concentrations of beta-adrenoceptors were restored to control levels at 12 months old in SHR and at 6 weeks after operation in 2K, 1C HT rats, although age-dependent decrease in beta-adrenoceptor was observed. The decrease in beta-adrenoceptor was associated with increase in plasma noradrenaline levels during the earlier stages of hypertension. But there is no correlation between beta-adrenoceptor concentrations and plasma noradrenaline levels in the chronic stages of hypertension. No significant difference was found in activities of 5'-nucleotidase, which is a marker enzyme of cell membrane, in membrane fractions between the hypertensive hearts and the controls, suggesting that the cardiac hypertrophy is not a determinant factor for change in beta-adrenoceptor. The observed decrease in beta-adrenoceptor concentration may reflect an increase in sympathetic nerve activity during development of hypertension. In the chronic stages of hypertension, additional factors may be involved in the restoration of beta-adrenoceptors.