We evaluated on the one hand, the vasorelaxant effect of camel and bovine casein tryptic hydrolysates (CTH and BTH, respectively) and their mechanism of action in thoracic aorta taken from 36 week old male Wistar Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) and, on the other hand, the putative antihypertensive effect of CTH and BTH in awake 36 week old male SHR. Relaxation to CTH and BTH was impaired in SHR thoracic aorta compared with that observed in WKY rat thoracic aorta. This relaxation was endothelium-dependent and mediated through the activation of the NO pathway in both rat strains. Oral administration of CTH (800 mg kg(-1) day(-1) per os for 15 days) decreased blood pressure and heart rate only during the first week of treatment in aged SHR. The same dose of BTH did not modify blood pressure and heart rate throughout the 15 days of treatment.