Carp [Cyprinus carpio) were acclimated to 8±1 C, 16±1.5 C and 26±1 C. Dark adapted retinas were isolated and light induced responses of HI horizontal cells recorded. The dynamic range of these cells was affected by temperature, showing a decrease on heating or cooling from an optimum temperature. The effect of acclimation was to shift this optimum in an adaptive manner. A move from 16 C to 8 C resulted in ~44% acclimation, while a move from 16 C to 26 C resulted in ~67% acclimation. The rates of change of membrane potential and latency of the response also showed adaptive changes on acclimation. Isolated horizontal cells were voltage clamped using the whole cell patch clamp technique. The current-voltage (I-V) relationship of the prominent anomalous rectifier current was displaced by changes in the extracellular potassium concentration and was blocked by Ba(^2+) or Rb(^+). Its amplitude did not appear to be affected by thermal acclimation. A pharmacologically isolated sustained Ca(^2+) current, with an I-V relationship characteristic of an L-type current, also showed no apparent thermal acclimation. The ratiometric calcium indicator Fura-2 was used to measure the intracellular calcium concentration in isolated horizontal cells. The intracellular calcium concentration rose on depolarization of the cells, in an extracellular calcium concentration dependent manner. This increase was blocked by various metal ions with varying sensitivities: La(^3+)>Cd(^2+)>Cu(^2+)>Co≥Ni(^2+). The rate of change of intracellular calcium concentration was increased by increased temperature, but did not appear to be affected by thermal acclimation. Sustained depolarizations (up to 15 minutes) resulted in sustained elevations in intracellular calcium concentration proportional to the degree of depolarization. Possible mechanisms underlying the long and short term effects of temperature on the horizontal cell responses are discussed. The sustained calcium current and the intracellular calcium concentration changes are disscused in terms of the potential roles of this current and the significance of the subsequent intracellular calcium concentration changes.