A noninvasive method of measuring hemoglobin flow through an organ by near-infrared spectroscopy (NIRS) is described that allows blood flow to be calculated. The method is derived from the Fick principle and uses a small change in arterial oxyhemoglobin concentration (brought about by a change in the fractional inspired O2 concentration) as an intravascular tracer. Changes in deoxyhemoglobin and oxyhemoglobin concentrations are quantified by monitoring variations in the absorption of near-infrared light in the organ, thus providing a measure of tracer accumulation. The tracer input function is calculated from the change in arterial O2 saturation, measured by pulse oximetry. The method was used to determine hemoglobin flow in the forearms of six healthy young adults on 10 occasions. Forearm hemoglobin flow ranged from 22.5 to 82.6 mumol.l-1.min-1. Calculated forearm blood flow ranged from 1.01 to 4.01 ml.100 g-1.min-1. For comparison, forearm blood flow was measured by venous occlusion plethysmography, and the relation between flow calculated by NIRS (y) and plethysmography (x) was y = 0.93x + 0.30 (r2 = 0.95). The mean difference between the methods was 0.14 ml.100 g-1.min-1. The technique may be widely applicable.