Effects of a modest dose of morphine sulfate (1 mg/kg) on total body energy metabolism, body thermal status, and the plasma concentrations of certain electrolytes and metabolites were investigated in conscious chronically instrumented pigs (n = 8). Control pigs (n = 8) received an equivalent volume of normal saline. Intravenous morphine injection led to an excitatory state associated with significant (P less than or equal to 0.05) immediate increases in O2 consumption. CO2 production, respiratory exchange ratio, and plasma concentrations of lactate, glucose, potassium, phosphate, epinephrine, and norepinephrine. Significant more gradual increases were observed in rectal and skin temperatures, body heat content, and the plasma concentrations of adrenocorticotropic hormone, cortisol, and phosphate. The hypermetabolic state persisted for approximately 1 h. Thereafter, most functional variables regressed toward, but did not reach, control levels. Increased muscle activity appeared to be the major factor underlying the rise in energy metabolism. Body heat storage after morphine injection appeared to be attributable to increased heat production coupled with an inadequate rise in heat loss.