Brown adipose tissue (BAT) is characterized by the existence of a unique mitochondrial protein (uncoupling protein or UCP) that uncouples oxidative phosphorylation and thus allows heat production. Its role in thermogenesis has been emphasized in recent years in response to cold stress (nonshivering thermogenesis, NST) as well as to hyperphagia (diet-induced thermogenesis, DIT). The present work was a first attempt to determine whether varying nutritional conditions could affect UCP gene expression. Total RNA was isolated from interscapular BAT and hybridized with a cDNA probe for UCP. Changes in UCP mRNA level were studied in rats fasted and refed for various periods at 23 or 28 degrees C. A 2 d fast at 23 degrees C reduced UCP mRNA level, whereas refeeding increased it. A prolonged starvation (53 h) induced an unexpected rise in UCP mRNA, which was associated with a fall in body temperature. Increasing the ambient temperature to thermoneutrality (28 degrees C) suppressed the fall in body temperature as well as the rise in UCP mRNA, which could then be characterized as a cold-induced response. Under the same environmental conditions (28 degrees C), refeeding still triggered a sharp, though transient, increase in UCP mRNA, showing that DIT was dissociated from NST.