1. In previous studies regulation of the F1F0-ATPase of mitochondrial complex V (ATP synthase) has been demonstrated in rat cardiomyocytes, canine mycocardium and skeletal muscle from children. The aim of the present study was to examine regulation of ATP synthase in human myocardium in response to different metabolic states. 2. Biopsy material was obtained from 10 children undergoing cardiac surgery. Mitochondria in the post-nuclear supernatant were incubated under different metabolic conditions for 15 min and then broken by sonication. ATP synthase was measured spectrophotometrically using a coupled enzyme assay. 3. ATP synthase can be rapidly measured in sonicated preparations of heart mitochondria from children. We show that direct regulation at the level of ATP synthase occurs in these mitochondria. ATP synthase capacity is decreased in response to blocking of the respiratory chain by cyanide (mimicking anoxia) or uncoupling of mitochondria, falling to 76% and 66% of control values respectively. Upregulation of ATP synthase can be demonstrated in heart mitochondria when the calcium concentration in the incubation medium is increased to 5 microM (130% of control). 4. ATP synthase is actively regulated in heart mitochondria from children. The enzyme is upregulated in response to increased calcium. This transition may reflect the increased energy demand when cardiac workload is increased.