Abstract: A heater 10 for warming blood, plasma and other solutions flowing through an intravenous tube prior to its entry into a patient. The heater has a housing that has an elongated channel 17 which extends from one end of the housing to the other and which is sized to receive and releasably hold in intimate contact an elongated portion of an intravenous tube. The channel is formed with an elongated slot 18 against which is mounted a heating element H controlled by a control circuit 30 and powered by batteries B. The control circuit may energize the heating element continuously or cyclically with a regulated pulse width modulator U1 in response to sensed temperatures.