The CO2 exchange of a field-grown eastern white pine (Pinus strobus L.) in Michigan was monitored during the winter-spring periods of 1983 and 1985. Its light-saturated photosynthetic rate in ambient CO2 and temperature was depressed during the winter, but recovered very rapidly during the last week of April in both years. The depressed rates in the winter were not due solely to low air temperatures, since the rates remained low even on warmer days or when the air temperature was artificially increased. Photosynthetic activity was more highly correlated with soil temperatrue at 30 cm than with air temperature or soil temperature at shallower depths. However, photoperiod or endogenous factors could not be eliminated as potential controls on photosynthetic activity.