The present study investigated possible interactions between body temperature, lipolysis and thyrotropin (TSH), the only hormone with a documented lipolytic effect in vitro in newborn infants. Healthy infants were either nursed in the usual way (n = 18) or protected from a decrease in body temperature (n = 17) during the first postnatal hour. The infants' axillary temperatures were measured immediately after birth and after 10 and 60 min. Blood samples were collected from the umbilical vein and from the infants 10 and 60 min after birth for analysis of TSH, glycerol, free fatty acids, 3-OH-butyric acid and glucose. We found that the mean (+/- SD) infant axillary temperature was 37.6 +/- 0.4 degrees C immediately after birth. In the routinely nursed infants, body temperature decreased to 37.0 +/- 0.5 degrees C at 10 min (p = 0.01) and to 36.6 +/- 0.4 degrees C at 60 min (p = 0.01); the cold-protected infants maintained their fetal temperature at 60 min of age. There was a four-fold increase in plasma TSH levels at 10 min, independent of the infant's body temperature, and the hormone level remained invariably high at 60 min. Plasma glycerol levels increased progressively at 10 min (p = 0.01) and 60 min (p = 0.01) in both infant groups, but were higher (p = 0.02) in the routinely nursed infants at 60 min. No significant relationship was found between TSH and glycerol levels. Infant body temperature did not affect the levels of free fatty acids, 3-OH-butyric acid or glucose. We conclude that the change in environmental temperature as a result of extrauterine adaptation causes thermal stimulation of the infant's body surface which leads to activation of the hypothalamic-pituitary TSH axis, resulting in maximal TSH release, and thus to induction of the lipolytic process. A decrease in body temperature may be an additive stimulus for further enhancement of lipolysis.