The purpose of this study was to elucidate the underlying mechanisms of blunted nocturnal blood pressure reduction in non-dipper hypertensive patients. We studied the diurnal variations in systemic hemodynamic indices and baroreflex sensitivity. In 45 subjects with essential hypertension (24 men; mean age, 49±1 years), intra-arterial pressure was monitored telemetrically. Non-dippers were defined as those with a nocturnal reduction of systolic blood pressure of less than 10% of daytime systolic blood pressure. Stroke volume was determined using Wesseling’s pulse contour method, calibrated with indocyanine green dilution. Baroreflex sensitivity was calculated as Δpulse interval/Δsystolic blood pressure on spontaneous variations. The mean values of the hemodynamic parameters were calculated every 30min. Twenty-six subjects were classified as non-dippers. Daytime blood pressure was not significantly different between dippers (149±4/87±3 mmHg) and non-dippers (147±3/82±2 mmHg), while the nighttime blood pressure was significantly reduced in dippers (131±3/77±2 mmHg) but not in non-dippers (145±3/80±2 mmHg). Nocturnal decreases in both cardiac index and stroke index were smaller in non-dippers (−12.0±1.2% and 1.5±1.0%) than in dippers (−17.5±1.4% and −2.2±1.1%). Baroreflex sensitivity significantly increased at nighttime both in dippers (6.5±0.6 to 8.0±0.7 ms/mmHg) and in non-dippers (5.1±0.3 to 6.4±0.4 ms/mmHg). Neither daytime nor nighttime baroreflex sensitivity was significantly different between the groups. We conclude that the hemodynamics of non-dipper essential hypertension are characterized by an inadequate nocturnal decrease in cardiac index and stroke index, suggestive of relative volume expansion or malsuppressed sympathetic activity. (Hypretens Res 2001; 24: 195-201)