To determine how changes in intravascular volume are sensed by atrial and renal volume receptors during pregnancy and the puerperium, circulating atrial natriuretic peptide (ANP) levels and plasma renin activity (PRA) were measured in conscious chronically catheterized rats on days 9-10, 15-16, 19-20, and 22 of pregnancy, on the first postpartum day, and in nonpregnant controls. Blood volume measured in a separate group of anesthetized rats increased progressively during pregnancy, and circulating ANP levels tended to decline, although not significantly below the nonpregnant value of 132 +/- 9 pg/ml (mean +/- SE). PRA remained similar to the nonpregnant value of 5.5 +/- 0.6 ng angiotensin I.ml-1.h-1 until day 22 of pregnancy, when it rose to 14.1 +/- 1.8 ng angiotensin I.ml-1.h-1 (P less than 0.001 vs. nonpregnant). In pregnant rats, PRA was suppressed after uninephrectomy and chronic administration of deoxycorticosterone and saline, demonstrating the ability of this hormonal system to respond appropriately to further increments in volume induced during pregnancy. On the first postpartum day plasma ANP rose to 268 +/- 26 pg/ml, and PRA fell to 3.6 +/- 0.4 ng angiotensin I.ml-1.h-1 (P less than 0.005 and P less than 0.05 vs. nonpregnant values, respectively). Thus it appears that the increased blood volume in normal pregnancy is not sensed by renal or atrial volume sensors, presumably because it is accommodated by an enlarged maternal vascular compartment. In the puerperium, however, due to the decreased size of the maternal vascular compartment, atrial and renal volume sensors recognize the intravascular volume as expanded.(ABSTRACT TRUNCATED AT 250 WORDS)