We studied the effects of intracoronary vasopressin on the relationship between pressure and flow in the coronary circulation of anaesthetised swine. In addition to measurements at control levels, diastolic pressure-flow relationships were constructed from steady-state points below a coronary pressure of 50 mmHg, where endogenous vasodilatation is strongly stimulated. At baseline pressures, flow fell 28% with maximal vasopressin effect. At all levels of diastolic pressure below 50 mmHg vasopressin also decreased flow, eg, at 30 mmHg flow was depressed by 40%. The slope of the steady-state pressure-flow relationship fell from 1.21 to 0.75 ml.min-1.mmHg-1. The diastolic pressure at which coronary flow ceased rose slightly from 13 to 15 mmHg. Intracoronary adenosine completely prevented vasopressin's effect, and the vasodilator response to adenosine was not attenuated by simultaneous administration of vasopressin. The porcine coronary circulation will constrict in response to vasopressin, not only at normal perfusion pressure, but also at low levels when metabolic vasodilatation is intense. Our study has implications about the therapeutic use of vasopressin, and demonstrates interaction of vasoactive stimuli in the coronary circulation.