Calcium (45Ca2+) efflux from preloaded synaptosomes was studied with a continuous perfusion technique and the rate constants of a two-phase efflux process calculated. When 16-Hz sinusoidally amplitude modulated 450-MHz microwave field (maximal incident intensity 0.5 mW/cm2, modulation depth 75%) was applied during the second phase, the rate constant increased by 38%. Unmodulated or 60-Hz modulated signals were not effective. This microwave field-induced change can be distinguished from CaCl2-stimulated 45Ca2+ efflux which is most probably derived intracellularly. These data suggest that the microwave-field induced change in calcium efflux probably did not involve intracellular calcium. Also, this change in the dynamic property of synaptosomes did not require gross anatomically intact tissue as a substrate for field-tissue interaction.