In 10 patients undergoing diagnostic cardiac catheterisation a bolus of 15 mg ST 567 was administered intravenously in 1.5 min followed by a 30 min infusion of 7.5 mg. The maximal plasma level was 343 +/- 131 ng ml-1 (mean +/- s.d.) 1 min after bolus injection and stabilised around 179 ng ml-1 thereafter. Heart rate decreased from 71 +/- 10 beats min-1 at baseline to 66 +/- 10 beats min-1 at the end of the bolus injection (-7%). This decrease in heart rate persisted during the whole observation period. Also there was an 8% reduction in peak positive first derivative of LV pressure. Cardiac output measured by thermodilution during atrial pacing decreased from 5.9 +/- 1.1 l min-1 to 5.3 +/- 0.7 l min-1 (P less than 0.02). In 3 patients with the largest decrease in cardiac output, the end diastolic LV pressure at the end of the observation period decreased, which may reflect a decrease in pre-load. Only in 1 patient the decrease in end diastolic LV pressure exceeded twice the standard deviation of the random error component of duplicate measurements. Thus, although normal therapeutic plasma levels were achieved, ST 567 demonstrated negative inotropic properties independent of changes in heart rate with this scheme of administration.