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The influence of a 6-week treatment of female Wistar rats with one-kidney, one clip (1-K.1 C) renal hypertension, with the calcium antagonist nifedipine on plasma volume, red cell Na+ handling and plasma atrial natriuretic peptide immunoreactivity (ANP-IR) was studied. Measurements were performed at 2 and 6 weeks after surgery.In 1-K.1 C rats plasma volume was increased and red cell Na+ pump activity was reduced only after 2 weeks. Blood pressure, heart weights and plasma ANP were massively increased after both 2 and 6 weeks. 1-K.1 C-rats treated with nifedipine had normal plasma volume, plasma ANP, and red cell Na+ pump activity in comparison with sham-operated rats. Increases in blood pressure and heart weights were attenuated. It is concluded that 1-K.1 C hypertension in the rat is associated with cardiomegaly, rise in plasma ANP, initial hypervolemia and depression of the membrane Na+ pump. Nifedipine prevents the occurrence of hypervolemia and secondary phenomena such as rises in plasma ANP and cardiomegaly. This may play an important contributory role in the prevention of pathological sequelae.