Warming increased the rate of fluid secretion (as measured using the Ramsay technique) by cAMP-stimulated Malpighian tubules of the tsetse fly Glossina morsitans morsitans Westw. The mechanism of this effect was explored by examining the temperature-induced changes in the current/voltage relationship, and in the equivalent electrical circuit, of isolated perfused tubules. Warming the unstimulated tubule from 20 to 30 °C induced only a reversible decrease in slope resistance, consistent with the known effect of temperature on the conductance of mixed electrolyte solutions. Tubules exposed to cAMP at room temperature showed a much larger fall in slope resistance, with little or no further fall on subsequent warming by 10 °C. In terms of the simple equivalent electrical circuit: (a) warming unstimulated tubules induced a fall in E Na (a measure of the electromotive force of the Na + transport mechanism), with no change in the calculated secretion rate; and (b) tubules exposed to cAMP at room temperature exhibited a 90% fall in series resistance ( R series), with no change in E Na, and doubling of the calculated secretion rate. On subsequent warming, R series remained unchanged, but both E Na and the calculated secretion rate increased markedly. Warming also tended to reduce the shunt resistance, which would contribute to the increase in secretion rate.