Paired determinations of the hydraulic conductivity (Lv) of individually perfused capillaries in frog mesentery were measured at 22-25 and 5-8 degrees C. The mean value of the ratio of the hydraulic conductivity at 22-25 to that at 5-8 degrees C was 1.76 +/- 0.22 (SE) in 10 capillaries perfused with albumin in the Ringer perfusate and 1.81 +/- 0.11 in six capillaries perfused with Ringer alone. The ratio of Lp can be accounted for in terms of a 1.60-fold increase in water viscosity between 22-25 and 5-8 degrees C. The results conform to the hypothesis that the structure of the principal pathway for water across the capillary wall is invariant with temperature; models of the water pathway that imply a change in the number of pathways or a change in pathway structure with temperature are incompatible with these results. Temperature gradients across the tissue are negligible in these experiments; calculations show that in the presence of temperature gradients, which occur during tissue cooling or rewarming, transcapillary water flows due to thermoosmosis are likely to occur.