The effects of temperature change (in vitro) on acid-base balance of skipjack tuna blood were investigated. By examining the relationship between blood pH and temperature (in vitro) under conditions of constant CO2 tension (open system), it was observed that dpH/dT = -0.013 U/degrees C. This value falls well within the range of in vivo values reported for other ectothermic vertebrates, and is only slightly different than results obtained in vitro under conditions of constant CO2 content (closed system; dpH/dT = -0.0165 U/degrees C). It is concluded that changes in pH following temperature changes can be accounted for solely by the passive, in vitro behaviour of the chemical buffer system found in the blood, so that active regulatory mechanisms of pH adjustment need not be postulated for skipjack tuna.