Young (5 week old) spontaneously hypertensive rats (SHR) were fed diets modified with respect to their calcium content. Control rats were given a normal calcium diet (0.3 per cent). During the experiment the systolic blood pressure (SBP), heart rate and serum level of total and ionized calcium were measured. Three diets were studied :calcium-free diet (0 per cent, h1), low calcium diet (0.03 per cent, h2) and high calcium diet (1.2 per cent, H). The h1 diet induced a transitory increase (at week 2), then a long-lasting decrease in SBP for 12 weeks. Heart rate and calcemia were significantly decreased. The h2 diet enhanced the increase in SBP and lowered heart rate for 10 weeks. Level of total and ionized serum calcium were unchanged. The high calcium diet (1.2 per cent) attenuated the increase in SBP for 44 weeks and enhanced the heart rate for 16 weeks. The serum level of total calcium remained stable but that of ionized calcium increased significantly at week 7. These data clearly establish that, in young SHR, a low calcium diet enhances the development of genetic hypertension and confirm earlier works obtained with calcium enriched diets. Experimental and clinical data lead us to emphasize the importance of alimentary calcium in the hypertensive pathology.