The aim of this study was to characterize hemodynamic, electrolytic and endocrine alterations produced by food restriction (50%) in pregnant rats for the purpose of evaluating the importance of these parameters on the plasma volume expansion and fetal growth. One hundred seventy six pregnant rats were divided into two groups, a control group (C) with an ad libitum diet and another with a restricted diet (U) (50% by weight of the diet of the control group). On days 5, 10, 15 and 20 of pregnancy, the weight of the mother, water intake, urine output, urine and plasma sodium concentration, plasma potassium concentration, blood pressure and heart rate, osmolality, plasma renin activity (PRA) and vasopressin were recorded. The number and weight of the fetuses were determined on days 15 and 20 of gestation. Food restriction results in inadequate weight gain in the mother and retardation of fetal growth. Water and sodium balance (p< or =0.001) were decreased in U group and basal PRA (p< or =0.001) was increased in U group. Food restriction did not significantly alter urine sodium excretion, plasma osmolality, plasma sodium and potassium values, blood pressure and basal vasopressin values. We conclude that the higher values of PRA, described in food restriction situations during pregnancy, seem to be caused by the adaptation to low sodium intake.