Changes due to hormone activity and caval compression by the uterus were noted in venous hemodynamics of the lower limbs during pregnancy. The purpose of the study was to search for a possible incidence of these modifications on venous inflow ant outflow during mercury strain gauge plethysmography (fig. 1). A total of 183 subjects were distributed into 4 groups: (table I) nonpregnant women of childbearing age (controls = C) and women of the first (G1), second (G2) and third (G3) trimesters of gestation. Women with a history of phlebitis or pronounced varicosis were excluded. The figures for venous filling (table II, III et IV) (delta V) were 3.2 (C), 3.1 (G1), 2.64 (G2) and 2.04 (G3). There was a significant (P less than 0.001) reduction in G3 (and left-sided G2) delta V compared to that of controls. Maximum venous outflow (MVO) was 125.3 (G1), 123.7 (G2) and 123.5 (G3), showing no significant difference with controls (125.1). It is concluded that no significant change in MVO occurred during pregnancy, whereas there was a decrease in delta approximately greater than V during the third trimester. These results suggest that the latter parameter should not be considered as a criterion for diagnosis of phlebitis in the pregnant woman.