The plasma electrolyte responses of the mammal and reptile on exposure to environmental temperatures from 0° to 37° were different. At high temperature (37°C) the activity of the mammal decreased, and plasma electrolyte levels rose in the first 12 hours but returned to normal by 1 week, despite a sustained increase in both haematocrit and plasma protein. At low temperatures (0°C) there was evidence of a falling urinary Na/K ratio, but little change in plasma electrolytes. The activity of the reptile was proportional to the environmental temperature over the range 0–37°C, but haematocrit, plasma sodium and protein levels remained very constant. Plasma magnesium bore a direct inverse correlation with temperature, and plasma calcium a similar, though less marked, response. Plasma potassium rose at higher temperatures in the reptile. In general, the poikilotherm responded to altered environmental temperature by calcium, magnesium and potassium adjustments, whereas the homoiotherm showed sodium and body fluid adjustments as recorded by protein and haematocrit changes.