Tubuloglomerular feedback activity was evaluated by micropuncture and microperfusion techniques in virgin and 12-day-pregnant Munich-Wistar rats. Plasma volume increases in pregnancy, which could suppress feedback activity, thus contributing to the rise in glomerular filtration rate observed in normal midterm pregnancy. Late proximal tubules were microperfused at 0, 10, 20, and 40 nl/min and the resulting filtration rate in the same nephron was evaluated. Nephron filtration rate (SNGFR) in proximal and distal tubules of other nephrons was also measured to assess the degree of activation of the tubuloglomerular feedback system and the relation of the spontaneous (normal) late proximal flow rate and SNGFR (distal tubule collections). SNGFR decreased significantly (from the 0 nl/min perfusion value) when late proximal tubules were perfused at 20 and 40 nl/min in both virgin and 12-day-pregnant rats. Tubuloglomerular feedback activity was not suppressed in pregnancy, but the relationship between SNGFR and late proximal tubule perfusion rate was reset for a higher value for SNGFR. The difference between proximal and distal SNGFR suggests that the feedback system was more activated in the virgin than in the pregnant rat. Thus, in spite of the known increases in plasma volume that occur in pregnancy, the kidney does not sense this volume expansion as a stimulus to suppress feedback activity.