The endothelin (ET) system is involved in regulation of myocardial function in health as well as in several diseases such as congestive heart failure, myocardial infarction and septic myocardial depression. Conflicting results have been reported regarding the acute contractile properties of endothelin-1 (ET-1). We therefore investigated the effects of intra-coronary infusions of ET-1 and of the ET_B-receptor selective agonist sarafotoxin 6c (S6c) with increasing doses in anaesthetized pigs. Myocardial effects were measured using analysis of left ventricular pressure-volume relationship. ET-1 elicited increases in myocardial contractile status (Ees 0.94±0.11 to 1.48±0.23, pre-load recruitable stroke work 68.7±4.7 to 83.4±7.2) that appear to be mediated through ET_A-receptors whereas impairment in left ventricular isovolumic relaxation (tau 41.5±1.4 to 58,1±5.0 and pressure half time 23.0±0.7 to 30.9±2.6) was ET_B-receptor dependent. In addition, intravenous administration of ET-1 impaired ventricular relaxation but had no effect on contractility. Intra-coronary S6c administration caused impairments in left ventricular relaxation (tau from 43.3±1.8 to 54.4±3.4) as well as coronary vasoconstriction. In conclusion, ET-1 elicits positive inotropic and negative lusitropic myocardial effects in a pig model, possibly resulting from ET_A- and ET_B-receptor activation respectively.