These experiments examine the interrelationships of luminal volume (V) and reabsorption (R) in the loop of Henle and arterial pressure (P). Re-collection micropuncture techniques were used to obtain samples from late proximal and early distal tubules before and after acute alteration of P in diuretic rats. In group 1, P was decreased from 140 ± 2 to 113 ± 3mm Hg with a servocontrolled aortic clamp between the renal arteries; flow into the loop (Q) decreased from 21.8 ± 0.8 to 16.9 ± 1.0 nl/min; V(t), estimated from mean flow through the loop and transit time, decreased from 6.0 ± 0.3 to 5.1 ± 0.3 nl; V(r), estimated from resistance to flow, also decreased from 3.5 to 2.8 nl; R decreased from 10.6 ± 0.5 to 9.4 ± 0.5 nl/min. In group 2, P increased from 121 ± 3 to 142 ± 3mm Hg with norepinephrine infusion (2 to 4 μg/kg.min), Q increased from 16.6 ± 0.7 to 22.5 ± 1.4 nl/min; V(t) was unchanged; and R increased from 9.4 ± 0.6 to 11.4 ± 1.1 nl/min. In group 3, when P was held constant with the aortic clamp, norepinephrine infusion decreased Q from 24.4 ± 0.9 to 20.3 ± 1.4 nl/min, left R unaltered, but decreased V(t) from 7.9 ± 0.3 to 5.5 ± 0.4 nl. R did not correlate significantly with V in any group. Conclusion. Norepinephrine decreases V(t) in the loop of Henle; V(t) does not appear to be a determinant of R from Henle's loop in these acute experiments. Acute variations of arterial pressure do not produce the correlations between R and V(t) observed in chronically hypotensive and hypertensive animals.