The direct assay of total renin (TRC) and active renin concentration (ARC) is a reality due to the availability of monoclonal antibodies against human renin. Because of this, a study has been performed in order to assess the circadian rhythmicity of TRC and ARC. The study was extended to plasma renin activity (PRA) and plasma aldosterone concentration (PAC) for a more complete assessment of the renin-angiotensin-aldosterone system (RAAS). Twelve clinically healthy subjects (6 males and 6 females, age from 20 to 25 years) volunteered for this study. Time-qualified data series were analysed by means of chronobiological procedures in order to validate the circadian rhythm and to correlate the sinusoidal profiles. The circadian rhythm was validated at a high significance for TRC, ARC, PRA and at a borderline significance for PAC. The periodic oscillations were significantly correlated, demonstrating that TRC, ARC, PRA and PAC cycles oscillate in synchronism during the 24-hour span.