The ventilatory response of green turtles (Chelonia mydas) to inspired CO2 was tested. Both immature (0.9--1.35 kg) and adult (59--130 kg) animals responded with increased ventilation (VE) that was primarily (immature) or exclusively (adult) due to increased respiratory frequency. The VE of the immature turtles while breathing air increased with temperatures between 15 and 35 degrees C with a Q10 of 1.95, but the peak VE of these turtles while breathing 4.5% CO2 in air was not significantly different at 15, 25 or 35 degrees C. VE increased irregularly with time throughout an hour of CO2 breathing in both groups, although several of the adult animals exhibited a first-breath response to CO2 and all animals increased VE within 5 min after the onset of CO2 breathing. The VE of an immature turtle at 35 degrees C increased with graded increases in CO2 up to 6%, but at 15 and 25 degrees C VE increased only up to 4.5% and decreased with 6% CO2. The results are discussed in terms of possible receptor mechanisms involved.