The present studies were undertaken to assess the relative effects of free fatty acid (FFA) availability, glucagon excess, and insulin deficiency on ketone body (KB) production in man. To determine whether an increase in FFA availability would augment KB production in the absence of insulin deficiency and glucagon excess, plasma insulin and glucagon were maintained at basal concentrations by infusion of somatostatin and exogenous insulin and glucagon, and plasma FFA were increased from 0.32 +/- 0.06 to 1.4 +/- 0.1 mM by a 2.5-h-infusion of a triglyceride emulsion plus heparin. KB production increased fivefold from 2.2 +/- 0.4 to 11.4 +/- 1.2 mumol . kg-1 . min-1, P less than 0.001. To determine whether insulin deficiency would further augment KB production, analogous experiments were performed but the replacement infusion of insulin was stopped. Despite a greater increase in plasma FFA (from 0.26 +/- 0.04 to 1.95 +/- 0.3 mM), KB production increased (from 1.5 +/- 0.3 to 11.1 +/- 1.8 mumol . kg-1 . min-1) to the same extent as in the absence of insulin deficiency. To determine whether hyperglucagonemia would augment KB production beyond that accompanying an increase in plasma FFA and, if so, whether this required insulin deficiency, similar experiments were performed in which the glucagon infusion rate was increased to produce plasma glucagon concentrations of 450-550 pg/ml with and without maintenance of the basal insulin infusion. When basal plasma insulin concentrations were maintained, hyperglucagonemia did not further increase KB production; however, when the basal insulin infusion was discontinued, hyperglucagonemia increased KB production significantly, whereas no change was observed in saline control experiments. These studies indicate that, in man, FFA availability is a major determinant of rates of KB production; insulin does not appear to influence ketogenesis rates by a direct hepatic effect, and glucagon can further augment KB production when FFA concentrations are increased but only in the setting of insulin deficiency.