Membrane currents were recorded in voltage-clamped oocytes of Xenopus laevis. Currents were produced in response to temperature jumps imposed by a heating lamp. Responses were larger when the animal (pigmented) hemisphere of the oocyte was illuminated as compared to the vegetal hemisphere; they arose because of a thermal effect as they were attenuated by removal of infrared wavelengths. The temperature jump responses comprised two distinct components. (i) a slow maintained current, which inverted direction at a membrane potential of about -25 mV and, (ii) a fast transient current, which at all potentials examined (-160 to +30 mV), was inward at the onset of a light flash and outward at the offset. The slow component probably arises through temperature-dependent changes in the `leakage' current of the oocyte, and measurements of reversal potentials in solutions of different ionic composition indicated that currents carried by Na$^+$ and H$^+$ ions contribute to the response. In contrast, the fast component was not altered by changes in composition of the bathing solution. This observation, together with the finding that the charge movements associated with the on and off transients were of similar magnitude, suggest that the fast current may arise because of the displacement of charges across the plasma membrane.