We inadvertently subjected a group of goats to 5 mo of cold exposure (mean minimum temperature less than 13°C) during an experiment designed to examine the effects of training by daily running on one member of each sibling pair. During the three coldest months, the sedentary but cold-exposed goats experienced a 34% increase in maximal oxygen uptake (O_2 max, P < 0.01) and a 29% increase in running speed at maximal (P < 0.05). When temperatures increased in the spring, both oxygen uptake and running speed decreased. We interpret these findings as evidence that cold is a sufficient stimulus to invoke the development of aerobic structures in muscle and that these structures subsequently can be utilized for the novel task of running. When the experiment was subsequently repeated without the cold exposure, running speed and O_2 max of trained animals increased less than in either group of cold-exposed animals. However, the cost of transport of these warm runners was lower than either group of cold-exposed animals (from 13-19%, P < 0.0001). Thus, although aerobic capacity was increased with acclimation to severe winter weather, cold-acclimated goats operated with lower efficiency during locomotion.