Abstract: The invention relates to a method for synchronizing an optical arterial pulse signal (2, 3, 4), obtained by passing electromagnetic radiation through a tissue, with a detected signal (1) obtained from heartbeat. The delays of optical arterial pulses in relation to corresponding heartbeats are used to set up a time lapse, during the course of which there must appear an arterial pulse acceptable in the determination of blood composition. The position of time lapse in relation to detected heartbeat is repeatedly determined while continuing measurements from the obtained optical pulses occurring during the time lapse.