Aldosterone has been reported to impair the baroreflex response in animal models. The present study aimed to investigate the acute effects of aldosterone on the autonomic nervous system and the baroreflex control of muscle sympathetic nerve activity (MSNA) and heart rate in healthy humans. Nine healthy subjects were examined in a double-blind, placebo-controlled, cross-over study design, receiving either i.v. aldosterone (100 μg) or placebo on the experimental day. Heart rate variability (HRV) was measured at rest, whereas blood pressure, heart rate and MSNA (assessed by microneurography from the peroneal nerve) were monitored both at rest and during baroreflex tests. Baroreceptor stimulation and deactivation was induced by i.v. infusion of incremental doses of phenylephrine and sodium nitroprusside. HRV indices at rest were specifically changed by aldosterone with a significant increase in standard deviation of RR intervals and total power, and a trend towards increased time domain parameters indicating parasympathetic predominance in heart rate regulation. Basal MSNA, blood pressure and heart rate remained unaffected by aldosterone administration. Sodium nitroprusside decreased diastolic blood pressure and increased MSNA as well as heart rate in both the aldosterone and placebo experiments. However, the tachycardic response to arterial baroreceptor deactivation was more pronounced in the aldosterone experiments. By contrast, baroreflex control of MSNA and heart rate during phenylephrine infusion was not affected by aldosterone. Thus, our study demonstrates that, in healthy humans, aldosterone tends to increase cardiac vagal activity and enhances the heart rate response to nitroprusside whereas MSNA remains unaffected.