The changes in the permeability properties of the rete capillaries of the eel in response to temperature shifts were studied during countercurrent perfusion at constant flow and pressure. Tracers and oxygen were added to the arterial perfusate. From the ratio of end concentrations of arterial to venous capillaries divided by surface area, calculated from rete weight, a value for the ratio of permeability to flow, P/F, with dimensions in centimeters−2was estimated. Because flow and surface area are constant, this provides an index of how permeability varies with time. A group of paracellular (albumin, sucrose, and sodium) and cellular (antipyrine, water, and oxygen) probes were used. When the temperature of the perfusate was raised abruptly from 25°C to 35°C, P/F values rose continuously and irreversibly from 0.042±0.009 to 0.281±0.112 cm−2(mean±SEM) for125I-albumin, from 0.082±0.006 to 1.74±0.070 cm−2for [14C]sucrose, and from 0.32±0.06 to 2.78±0.62 cm−2for22Na, whereas they were not modified for [14C]antipyrine, [3H]water, and 02. Gradual increase of temperature was accompanied by a smaller rise in sucrose and sodium permeability and no change in albumin permeability., with decrease, the change was reversible. When the temperature was lowered abruptly from 25°C to 5°C, the P/F ratio for sucrose, sodium, and oxygen did not change, while that for [3H]water and [14C]antipyrine decreased to plateau values, from 13.0±3.2 to 9.6±2.6 and from 7.9±2.2 to 4.4±0.4 cm−2, respectively, simultaneously, P/F values for125I-albumin increased from 0.030±0.007 to 0.063±0.012 cm−2Thus, increase in rete temperature primarily increases paraceilular permeation, whereas decrease in temperature primarily decreases permeability to [14C]antipyrine and [3H]water cellular probes. Oxygen is unaffected throughout.