Marine iguanas, Amblyrhynchus cristatus, held for 12-18 h at 16, 24 and 35 degrees C exhibited an arterial pH change of -0.001 delta pH/delta degree C. The arterial pH changed by -0.004 delta pH/delta degree C between 16 and 24 degrees C and by -0.015 delta pH/delta degree C between 24 and 35 degrees C. When the animals were allowed to cool to 16 degrees C and remarm to 35 degrees C after 12-18 h at 35 degrees C, the pH change was -0.015 delta pH/delta degree C. Arterial pH measured during warming to 35 degrees C after 12-18 h at 16 degrees C was relatively constant at around pH approximately equal to 7.60 returning slowly toward the 35 degrees C equilibrijm pH approximately equal to 7.44. An increase in VE/MCO2 (L . mmol-1) is seen with decline in equilibrium body temperature from 0.79 at 35 degrees C to 1.66 at 16 degrees C. The ventilatory response is associated with a fall in PaCO2 (24 Torr, 35 degrees C; Torr, 16 degrees C). Arterial CO2 content and [HCO3-] remain relatively constant. The observed arterial pH-body temperature relationshi is similar to other reptiles; however, thermal history appears to influence the relationship. Marine iguanas probably experience only brief periods of time at body temperature as low as 18 degrees C.