Hydrostatic pressures decreased in the dorsal aorta (from 21.6 +/- 2.0 to 18.7 +/- 0.2 mmHg), proximal (from 19.5 +/- 1.6 to 16.2 +/- 1.7 mmHg) and distal (from 16.7 +/- 0.7 to 13.2 +/- 0.9 mmHg) renal arteries, and glomerular capillaries (from 16.1 +/- 1.1 to 12.3 +/- 0.6 mmHg) of anesthetized lampreys transferred from freshwater to isosmotic 20-30% seawater. Maximal vascular resistance appeared to be in the efferent arteriole; there was a 67% decrease in pressure between glomerular and peritubular capillaries. Plasma oncotic pressure was unchanged. The calculated afferent effective filtration pressure decreased by 87% after transfer and showed a good correlation with single nephron filtration rate. Effective renal plasma flow was high but variable in freshwater lampreys and decreased by 84% after transfer, but glomerular filtration rate did not decrease proportionately and there was a nonsignificant increase in mean filtration fraction from 0.045 +/- 0.022 to 0.080 +/- 0.021. Calculation of glomerular efferent oncotic pressure indicated that filtration equilibrium did not exist in freshwater lampreys but was attained after transfer. The mean coefficient of filtration of freshwater lampreys was 0.028 +/- 0.002 nl X s-1 X mmHg-1.