Adequate plasma volume (PV) and extracellular fluid volume (ECF) are essential for blood pressure and fluid regulation. We tested the hypotheses that combined progesterone-estrogen (P₄-E₂) administration would increase ECFV with proportional increases in PV, but that progesterone (P₄) would have little independent effect on either PV or ECFV. We further hypothesized that this P₄-E₂-induced fluid expansion would be a function of renin-angiotensin-aldosterone system (RAAS) stimulation. We suppressed progesterone and estrogens with a gonadotropin releasing hormone antagonist in 8 women (25±2 y) for 16 days; progesterone (200 mg/day) was added for days 4-16 (P₄) and 17 -estradiol (2 x 0.1 mg/day patches) for days 13-16 (P₄-E₂). On days 2 (GnRH antagonist), 9 (P₄) and 16 (P₄-E₂) we estimated ECF and PV. In order to determine the rate of protein, and thus water movement across the ECF, we also measured transcapillary escape rate of albumin (TER_alb). In P₄, P_[P4] increased from 2.5±1.3 to 12.0±2.8 ng/ml (P<0.05) with no change in P[_E2], (21.5±9.4 to 8.6±2.0 pg/ml). In P₄-E₂, P_[P4] remained elevated (11.3±2.7 ng/ml) and P_[E2] increased to 254.1±52.7 pg/ml (P<0.05). PV was increased during P₄ (46.6±2.5 ml/kg) and P₄-E₂ (48.4±3.9 ml/kg) compared to GnRH antagonist (43.3±3.2 ml/kg, P<0.05), as did ECF (206±19, 244±25, 239±27, ml/kg, P<0.05 for GnRH antagonist, P₄, P₄-E₂ and, respectively), although TER_alb was lowest during P₄-E₂ (5.8±1.3, 3.5±1.7 and 2.2±0.4 %/hr for GnRH antagonist, P₄ and P₄-E₂, respectively, P<0.05). Serum aldosterone (S_[ALD]) increased during P₄ and P₄-E₂ compared to GnRH antagonist (79±17, 127±13 and 171±25 pg/ml for GnRH antagonist, P₄ and P₄-E₂, respectively, P<0.05), but plasma renin activity (PRA) and P_{[ANG II]} were only increased by P₄-E₂. This study is the first to isolate progesterone effects on ECF, however the mechanisms for the ECF and PV expansion have not been clearly defined.