This communication describes a spectrophotometric device which determines the arteriovenous oxygen difference (a-v O2) on whole blood flowing through optical cuvettes. The instrument consists entirely of commercially available solid-state components, including the light sources, light-emitting diodes. When calibrated against conventional determinations of blood oxygen content, the output of the a-v O2 analyzer was shown to be highly linear and to be independent of total hemoglobin concentration. The electronic circuit, the theory of the measurement, and sources of error are described in detail.