Softshell turtles overwinter in the same bodies of water as some emydids, but their reduced shell and increased non-pulmonary gas exchange may contribute to a different mechanism of overwintering. The dynamics of bimodal respiration, diving behaviour and blood acid-base status in Apalone ferox and Chrysemys picta were investigated under two different temperatures combined with three different aquatic P(O2) levels. Both species obtained oxygen through pulmonary and non-pulmonary routes. Apalone ferox obtained more oxygen through non-pulmonary routes and increased its non-pulmonary V(O2) in response to both higher aquatic P(O2) and lower temperatures. Both species increased pulmonary V(O2) in response to higher temperatures. As a consequence of the greater reliance of A. ferox on pulmonary V(O2), warmer temperatures caused plasma P(CO2) and [HCO(3)(-1)] values to increase significantly compared with C. picta. Apalone ferox, which is efficient at bimodal respiration, displayed a high degree of plasticity with respect to both its respiratory and acid-base profiles, behaving more like an aquatic air-breathing fish in bimodal respiration at low temperature and more like a terrestrial air-breather at high temperature. Chrysemys picta, which is poor at bimodal respiration, was highly dependent on aerial gas exchange at both temperatures. Aquatic P(O2) did not change any of the behavioural variables measured. At warm temperatures, A. ferox met O(2) demands by increasing the rate of lung ventilation, which resulted in a significantly greater number of breathing bouts per hour and breaths per emersion period. However, the number of breaths per bout was not affected by temperature. As temperatures decreased, A. ferox utilized its non-pulmonary respiration ability and significantly increased its dive duration. Apalone ferox became less active at colder temperatures by significantly increasing the duration of inactive periods (from 4 to 18 min) and by significantly decreasing the frequency of activity bursts. Chrysemys picta also met the higher gas-exchange demands associated with increased temperature by increasing the rate of lung ventilation; however, this increase was not as large as that measured in A. ferox. Chrysemys picta displayed multiple rhythmic breaths per bout. These results indicate that, unlike aquatic P(O2), temperature is an important factor in the regulation of diving and ventilatory behaviour in turtles. The species responded to temperature in dissimilar ways because of differences in their bimodal respiration ability.