Measurement of the rate of temperature change in muscle as ambient temperature varies allows blood flow to be calculated. The procedure is simplified by taking the temperature measurements in deeply sited tissues of large fish where convection by the blood is the dominant heat-transfer process and heat transfer by conduction is minimal. Rates of blood flow in free-swimming swordfish, Xiphias gladius, and blue sharks, Prionace glauca, have been obtained by using acoustic-telemetry techniques to measure water temperature and muscle temperature. Blood-flow rates of 34 ml · kg⁻¹ · min⁻¹ for the blue shark and 15 ml · kg⁻¹ · min⁻¹ for the swordfish are generally comparable to blood-flow rates measured in other fish via a variety of techniques. Temperature effects are considered and systematic variations of blood flow in the blue shark are related to changing activity as the fish ascends and descends.