1. The absolute sensitivity of vision was studied as a function of temperature in two species of frog (Rana temporaria, 9-21 degrees C, and Rana pipiens, 13-28 degrees C). 2. Log behavioural threshold (measured as the lowest light intensity by which frogs trying to escape from a dark box were able to direct their jumping) rose near-linearly with warming with a regression coefficient of 1.26 +/- 0.03 log units per 10 degrees C (Q10 = 18). Threshold retinal illumination corresponded to 0.011 photoisomerizations per rod per second (Rh* s-1) at 16.5 degrees C. 3. The effect of dim backgrounds on jumping thresholds suggested 'dark lights' of 0.011 Rh* s-1 at 16.5 degrees C and 0.080 Rh* s-1 at 23.5 degrees C, corresponding to Q10 = 17. 4. Response thresholds of retinal ganglion cells were extracellularly recorded in the isolated eyecup of R. temporaria. The thresholds of the most sensitive cells when stimulated with large-field steps of light were similar to the behavioural threshold and changed with temperature in a similar manner. 5. The decrease in ganglion cell 'step' sensitivity with warming consisted of a decrease in summation time (by a factor of 2-3 between 10 and 20 degrees C) and an increase in the threshold number of photoisomerizations (a decrease in 'flash' sensitivity, by a factor of 2-5 over the same interval). No effect of temperature changes on spatial summation was found. 6. Frequency-of-response functions of ganglion cells indicated an 11-fold increase in noise-equivalent dark light between 10 and 20 degrees C (mean values in four cells 0.009 vs. 0.10 Rh* s-1). 7. The temperature dependence of ganglion cell flash sensitivity could be strongly decreased with dim background illumination. 8. It is concluded that the desensitization of dark-adapted vision with rising temperature is a retinal effect composed of shortened summation time and lowered flash sensitivity (increased numbers of photons required for a threshold response) in ganglion cells. The desensitization bears no simple relation to the apparent retinal noise increase.