Eastern red-spotted newts are ectotherms, aquatic as adults, and active year-round, breeding even during winter under ice. Earlier research, with field-captured newts, showed a correlation between seasonal changes in the activity of some muscle metabolic enzymes and in the behavior of the newts in a thermal gradient. This study was undertaken to further characterize acclimatory responses in the newts in a more controlled laboratory environment. Newts were obtained during fall, and maintained at 15 degrees C with 12:12 LD for 4 weeks for SMR (at 8 and 26 degrees C) and temperature preference experiments. Subsequently, half the newts were exposed to summer conditions (26 degrees C, 14:10 LD) and half to winter conditions (8 degrees C, 10:14 LD). After 12 weeks, SMR and temperature preference experiments were repeated, and enzyme assays for cytochrome c oxidase (CCO), citrate synthase (CS), and lactate dehydrogenase (LDH) were performed on muscle tissue homogenates, also at 8 and 26 degrees C. Newts changed all three parameters in the laboratory. SMRs were highest in winter-acclimated newts and lowest in summer-acclimated newts, whereas temperature preference was lowest in winter-acclimated newts and highest in summer-acclimated newts. Finally, CCO activity was completely compensated in winter-acclimated newts, CS activity was partially compensated, and LDH activity was not seasonally sensitive. These results indicate a connection or relationship between changes in seasonal environmental conditions, and some aspects of the muscle biochemistry, SMR, and thermoregulatory behavior of these ectotherms.