1. Release of noradrenaline from the perfused cat spleen, or from isolated spleen slices, in response to prolonged nerve stimulation or maintained depolarization by potassium was measured. 2. Prolonged stimulation of the splenic nerves at 2, 10 and 30 Hz for 10 min evoked release, which was maximum during the first 2 min, and then declined during the remaining period of stimulation. When noradrenaline release was induced by high potassium from the perfused spleen or from isolated slices, it followed a similar time course to nerve stimulation. Similar results were obtained from phenoxybenzamine-treated spleens, using both modes of stimulation. 3. Stimulation of the splenic nerves in calcium-free Krebs solution did not release noradrenaline. If calcium was introduced at a later stage during stimulation, the release was markedly diminished. In phenoxybenzamine- or phentolamine-treated spleens, stimulation of the nerves in the presence of calcium evoked a secretory response which was comparable to the one produced by introduction of calcium after a few minutes of nerve stimulation. 4. Simultaneous application of calcium plus high potassium always produced a much greater secretion of noradrenaline than application of calcium after a few minutes of potassium depolarization. Release of noradrenaline by potassium from phenoxybenzamine-treated spleens was also much greater if calcium and potassium were added simultaneously than addition of calcium after a few minutes of potassium depolarization. 5. In the presence of maintained depolarization by potasssium, tyramine was effective in causing release of noradrenaline.