In normal hearts, the critical value of the endocardial viability ratio (EVR) is thought to be less than 0.5. As myocardial regional dysfunction is a sensitive index of subendocardial ischaemia, the relationship between EVR and regional function has been studied in an experimental model of coronary artery constriction. In 13 dogs anaesthetized with halothane (0.5-2.0% inspired concentration), diastolic and systolic pressure time indices were obtained by planimetry, and their ratio (EVR) correlated with regional function. Halothane alone caused a significant reduction in EVR from 1.38 +/- 0.08 to 1.15 +/- 0.04 (mean +/- SEM). In the presence of coronary artery constriction a similar decrease in EVR was observed and was accompanied by post-systolic shortening (PSS), an indicator of regional dysfunction. At high concentrations of halothane, there was an inverse correlation between reduction in EVR and increase in PSS. Mean EVR of approximately 0.9 (mean = 0.92 +/- 0.02) was associated with significant worsening of regional function.