The Langendorff heart preparation was used to investigate the release of ATP from the guinea pig heart in response to increased coronary flow. The luciferin-luciferase firefly technique was used to determine levels of ATP in the perfusate. During periods of increased flow, ATP release was rapidly and significantly increased by at least tenfold the basal release. The pressure/flow ratio was reduced in increased-flow conditions suggesting that coronary vasodilatation had occurred. It is concluded that ATP release from the guinea pig heart is increased under raised-flow conditions. It is suggested that this ATP release induces coronary vasodilatation.