A blood constituent measuring apparatus and method, which can be used to measure the oxygen (or other blood constituent) content of the blood. The apparatus includes sources of two or more wavelengths of light for transmitting, e.g., red and infrared light through a portion of the body, and a photodetector for generating respective signals representing each wavelength of light transmitted through the body portion. The photodetector signals have pulsatile and non-pulsatile components. The oxygen content of the blood is computed based on the light transmitted through the body portion at each wavelength, as determined from the pulsatile component, amplified alone after the much larger non-pulsatile component is subtracted from it. The apparatus can compute the oxygen content of patients with weak pulses, or unstable physiological states, or both, by using, preferably, a plurality of independently settable gains, to maintain the signal level within a range suitable for accurate measurement. In addition, the apparatus preferably compensates for drift in the non-pulsatile component which can be caused when the patient's blood pressure, for example, becomes erratic, thereby increasing the accuracy of its computation of the oxygen content of the blood. To the extent possible, the signals produced for the different wavelengths are time-multiplexed onto a single channel. A test mode is preferably provided, in which only one wavelength of radiation is produced, resulting in a computation equal to what would be obtained in normal operation if the amounts of sensed radiation were the same for all wavelengths.