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During surgery associated with rapid blood loss, rapid fluid administration and transfusion of packed red blood cells, measurement of total hemoglobin concentration provides necessary information to direct decisions regarding transfusion. However, measurement of total hemoglobin concentration currently requires collection of a blood sample and either transport to a laboratory or centrifugation at the bedside. Recently we proposed to use optoacoustic technique for noninvasive and continuous monitoring of total hemoglobin concentration. High resolution of the optoacoustic technique may provide accurate measurement of total hemoglobin concentration by detecting and analyzing optoacoustic signals induced by short optical pulses in blood circulating in arteries or veins. We designed, built, and tested in vitro and in preliminary in vivo experiments a portable optoacoustic system for monitoring of total hemoglobin concentration in the radial artery. The system includes a nanosecond laser operating in the near-IR spectral range and a sensitive optoacoustic probe designed for irradiating the radial artery through the skin and detecting optoacoustic signals induced in blood. Results of our studies indicate that parameters of optoacoustic waves induced in blood are dependent on total hemoglobin concentration. The data suggest that the optoacoustic system may be used for accurate, noninvasive, and continuous monitoring of total hemoglobin concentration.