In 5 unanesthetized dogs, previously trained to lie quietly, we have studied the patterns of heart rate (FC), arterial pressure (PA), mesenteric blood flow (FM) and mesenteric resistance (RM = PA/FM) during emotional stress induced by a sudden loud noise. The experiments have been repeated after chronic sympathetic blockade with guanethidine. Mean values of FC, PA and RM have been calculated during a 20 sec control period, and second by second during a 30 sec period after the stimulus. The FC rises significantly and rapidly during the first 10 sec after the stimulus both in control and in treated dogs; in the latter, however, the rise in the first 7-8 sec after the stimulus is significantly smaller. In the control dogs RM and PA rise considerably together with FC. In the treated dogs RM and PA rise about 10 sec after the stimulus, probably as an effect of adrenal catecholamines. It is suggested that during the first 10 sec after the stimulus the rise of FC is only partially due to a sympathetic action, a significant role being played by parasympathetic inhibition.