Acute effects of the antilipolytic agent p-chlorophenoxyphenoxyisobutyrate (CPIB) on net myocardial uptake of free fatty acids (FFAu), myocardial oxygen consumption (MVO2), and mechanical activity of the heart were studied in seven intact, anesthetized dogs. CPIB was given i.v and studies performed under basal conditions and during isoprenaline-stimulated lipolysis. Under basal conditions CPIB did not influence FFAu, MVO2 or mechanical activity of the heart. During isoprenaline infusion CPIB reduced FFAu from 42.3 +/- 15.6 (mean +/- SEM) to 15.6 +/- 8.3 micromol/min . 100 g tissue (p less than 0.02) and MVO2 from 28.2 +/- 3.3 to 23.8 +/- 2.5 ml/min . 100 g (p less than 0.02). The reduction in MVO2 could not be explained by reduced mechanical activity of the heart. Most likely the mechanism for the reduction in MVO2 effected by CPIB during isoprenaline infusion was mediated by reduced myocardial FFA consumption.