Spontaneously hypertensive rats (SHR) have an activated brain angiotensin system. We hypothesized 1) that ventilation () would be greater in conscious SHR than in control Wistar-Kyoto (WKY) rats and 2) that intravenous infusion of the ANG II-receptor blocker saralasin would depress respiration in SHR, but not in WKY. Respiration and oxygen consumption (O₂) were measured in conscious aged-matched groups (n = 16) of adult female SHR and WKY. For protocol 1, rats were habituated to a plethysmograph and measurements obtained over 60-75 min. After installation of chronic intravenous catheters, protocol 2 consisted of 30 min of saline infusion (~14 µl · kg¹ · min¹) followed by 40 min of saralasin (1.3 µg · kg¹ · min¹). , tidal volume (VT), inspiratory flow [VT/inspiratory time (TI)], breath expiratory time, and O₂ were higher, and breath TI was lower in "continuously quiet" SHR. In SHR, but not in WKY rats, ANG II-receptor block decreased , VT, and VT/TI and increased breath TI. During ANG II-receptor block, an average decrease in O₂ in SHR was not significant. About one-half of the higher in SHR appears to be accounted for by an ANG II mechanism acting either via peripheral arterial receptors or circumventricular organs.