The effects of a pressor dose of angiotensin II on the respiratory pattern of chloralose anaesthetized mongrel dogs was studied. In addition, since the hypertensive action of angiotensin II stimulates baroreflex pathways, the effects on breathing of baroreceptor stimulation were examined using aortic balloon inflation and i.v. phenylephrine. In the baroreceptor-denervated dog, i.v. angiotensin II was associated with a reduction in both inspiratory and expiratory time while both inspiration and expiration proceeded more quickly. Thus breathing became deeper and more rapid. Most of these effects of angiotensin II were opposite to the effects of hypertension caused by phenylephrine and aortic balloon inflation. These two forms of hypertension caused an increase in inspiratory and expiratory time, a decrease in the rate at which inspiration proceeded and a fall in tidal volume. These effects were not seen in baroreceptor denervated animals. In the intact dog angiotensin II caused a marked vagal dependent fall in tidal volume and inspiratory time while the rates at which inspiration and expiration proceeded were unaffected. It is concluded that angiotensin II has three independent effects on respiration. These are a respiratory depressant effect due to stimulation of baroreflex pathways by its pressor action, a 'central' stimulatory effect and an effect on the vagal control of inspiratory time.