Histofluorescent and quantitative microfluorimetric studies have been performed on glomus cells of the rat carotid body, which fluoresce intently after treatment by paraformaldehyde vapor. Reserpine causes a reduction in intensity of fluorescence of about 90%. Subcutaneous injections of calcium chloride (100-300 mg/kg) were given before reserpine. Calcium prevents the depletion of catecholamies from the glomus cells by reserpine. This effect is dose related in that the depletion of catecholamines by heavy reserpine doses (15 mg/kg) cannot be overcome by calcium, medium doses (5 mg/kg) can be overcome but variably, and light doses (1 mg/kg) are always overcome substantially and can result in virtually complete inhibition of the depletion by reserpine. If calcium might counteract the effects of reserpine by occupying attachment sites of the vesicular membrane, thereby preventing reserpine from reaching its site of action, resulting in the usual uptake of catecholamines by the vesicles and suppression of the depleting action of reserpine.