After earlier studies in which secretion of aldosterone was demonstrated to be important in rat arterial smooth muscle cell (RASMC) proliferation in vitro, the presence of both 11beta-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2) gene transcription were shown in these cells by real-time reverse transcription-polymerase chain reaction (RT-PCR). In proliferation studies, tritiated thymidine incorporation into RASMC and RASMC cell number were both significantly increased by angiotensin II (Ang II) (10(-7) mol/L) compared with controls (P<0.01), but this effect was inhibited by the 3beta-hydroxysteroid-dehydrogenase inhibitor trilostane (10(-6) mol/L and 10(-5) mol/L, P<0.05). Aldosterone alone added to RASMC did not significantly change tritiated thymidine incorporation when compared with controls, but the Ang II-induced increase was significantly enhanced by aldosterone at 10(-10) mol/L and 10(-8) mol/L (P<0.05). Neither corticosterone nor 18-hydroxydeoxycorticosterone had any such potentiating effect. RT-PCR analysis and real-time quantitative RT-PCR revealed an increase of Ang II type-1 (AT1) receptor mRNA in RASMC treated by aldosterone (10(-8) mol/L) compared with untreated controls, and this was correlated with a small but significant increase in AT1 receptor protein (P<0.05), as assessed by immunoblotting analysis. These data confirm that steroid production by RASMC is critical in the response to Ang II, and the data support the view that aldosterone specifically is required for the full proliferative response to Ang II in RASMC. One way it may act is by modulating the expression and functions of the AT1 receptor.