To evaluate the effect of fluctuations in environment and body temperatures on preterm infants, we recorded these variables in very immature newborn infants (birth weight less than 1000 gm) cared for in double-walled incubators (Air-Shields model C-100 and Ohio model IC). Both incubators maintained environmental temperatures corresponding overall to the set point, despite incubator openings. Under skin temperature servocontrol, however, environmental temperature fluctuations were greater than 2 degrees C even in strictly controlled conditions. The pattern of incubator temperature fluctuations depended on the set point rather than on the type of incubator (conventionally heated or heated by warm air blown between the double walls). The long-term clinical significance of the incubator temperature variability remains to be determined; the choice between air and skin servocontrolling should depend in part on the need for environmental stability.