Despite long-standing observations of a whole-body thermogenic effect of glucagon, the role of glucagon in activating thermogenesis in brown adipose tissue has not often been studied. We investigated the ability of administered glucagon to produce alterations in brown adipose tissue similar to changes produced by accepted stimuli of brown fat activity: cold, norepinephrine, and overfeeding. Eighteen days of glucagon injections (1 mg/kg) to male Sprague-Dawley rats produced, relative to saline-injected controls, decreases in feed efficiency and increases in brown adipose tissue weight, protein content, DNA content, and mitochondrial mass as reflected in cytochrome oxidase activity. The observed changes were similar, though of lesser magnitude, to changes produced in these same parameters induced by administration of norepinephrine (250 micrograms/kg) for a positive control group. Four days of glucagon administration (1 mg/kg) produced increases in specific activity of cytochrome oxidase and lipoprotein lipase. After 8 days of glucagon administration, changes in whole-pad activity similar to those seen with 18 days of administration were present. Glucagon also increased whole-pad lipoprotein lipase activity after 4 and 8 days. Surgically denervated interscapular brown adipose tissue retained its ability to respond to exogenous glucagon, though the magnitude of the response was diminished. Guanosine 5'-diphosphate (GDP) binding to brown adipose tissue mitochondria was measured as an assessment of functional state after 5 days of glucagon (1 mg/kg). There was an increase in GDP binding relative to controls whether expressed as picomoles per milligram mitochondrial protein or nanomoles per pad.(ABSTRACT TRUNCATED AT 250 WORDS)