Previous studies have shown that a chemically reactive analog of prazosin, SZL-49, reduces the alpha-1 adrenoceptor population by a maximum of 60% (Kusiak et al., 1989). These data support the idea that alpha-1 receptor subtypes exist and only one is sensitive to alkylation by SZL-49. In the present study male rats were injected (i.p.) with SZL-49 (0.5-30 mg/kg) and the effects on [3H]prazosin binding, systemic arterial blood pressure and the pressor response to phenylephrine were assessed 24 hr later. SZL-49 treatment decreased the number of [3H]prazosin sites without affecting receptor affinity. The maximal reduction in binding sites was 60% (32 fmol/mg). At doses of 0.5-, 1-, 5- and 10-mg/kg SZL-49 reduced the receptor number to the same level (83 fmol/mg control; 47-60 fmol/mg treated). Injection of SZL-49 had no effect on resting blood pressure. However, drug treatment (0.5 mg/kg and greater) shifted the phenylephrine dose-response curve to the right. The maximal 1-, 5- and 10-mg/kg SZL-49, the ED50 for phenylephrine was approximately the same. Therefore, over a 20-fold range of SZL-49 concentrations no further reduction in receptor number or phenylephrine ED50 was observed. These data support the idea that S2L-49-sensitive and resistant alpha-1 receptors exist in vivo. The alkylation resistant receptor is capable of maintaining resting blood pressure. Furthermore, both receptors appear to be involved in mediating the increase in blood pressure observed with phenylephrine.