The effects of sympathetic nerve stimulation on beat rate, force, intracellular Ca²⁺ concentration ([Ca²⁺]_i) measured using fura 2, and membrane potential were recorded from the spontaneously beating toad sinus venosus. Short trains of stimuli evoked an increase in the beat rate and force. During this tachycardia the amplitude of pacemaker action potentials was not changed, but there was an increase in the basal level of [Ca²⁺]_i with little change in peak [Ca²⁺]_i measured during each action potential. Depletion of intracellular Ca²⁺ stores with caffeine (3 mM) abolished all responses to sympathetic nerve stimulation. The effects of caffeine were fully reversible. Caffeine (3 mM), in the presence of the Ca²⁺-ATPase inhibitor thapsigargin (30 µM), abolished irreversibly the chronotropic and inotropic responses evoked by sympathetic nerve stimulation. Ryanodine (10 µM) attenuated, but did not abolish, these responses. These results suggest that, in the toad sinus venosus, increases in force and beat rate evoked by sympathetic nerve stimulation result from the release of Ca²⁺ from intracellular Ca²⁺ stores.