Plasma free fatty acid (FFA) levels and myocardial accumulation of lipid droplets were studied in mice receiving 85 mg isoproterenol (ISP)/kg b.wt. with and without pretreatment with nicotinic acid (NA) 30 min before ISP injection, and in saline-injected controls. The animals were killed 30 min after the ISP injection. Plasma FFA were determined by a radiochemical method. Lipid droplet accumulation was analyzed by quantitative stereologic techniques. Plasma FFA were raised from 505.8 +/- 15.7 to 1,808.8 +/- 87.8 mumol/l after ISP treatment and to 1,108.0 +/- 34.7 mumol/l in the group receiving NA pretreatment. However, NA pretreatment did not bring about a corresponding decrease in the fractional volume of lipid droplets, as both groups showed an increase of five times that of the controls. These results are discussed in terms of the pathogenesis of ISP-induced lipid accumulation.