In the trp mutant of Drosophila the receptor potential decays close to baseline during prolonged illumination (Fig. 1). The mutant can be characterized by two parameters: the rate of decay of the receptor potential towards the dark baseline during illumination (decay rate) and the recovery time of the responses in the dark following prolonged illumination (dark recovery). These parameters may be modulated consistently: rearing the fly at 19 °C instead of 25 °C slows the decay rate but speeds up an initial phase of the dark recovery when measurements are performed at 25 °C (Figs. 1, 2). A transition from one phenotype to the other and vica versa is observed within a few days when the adults are transferred from one temperature to the other (Fig. 3). This suggests turnover of the defective gene product.