The effects of nitroglycerin, infused intravenously at 3.4 and 7.5 micrograms/min over 30 min, on haemodynamic parameters were determined in the morning and the afternoon in a randomized, placebo-controlled study in 5 healthy volunteers. The mean steady-state concentrations of nitroglycerin reached in the plasma during the infusions of 3.4 and 7.5 micrograms/min were 0.35 +/- 0.06 ng/ml and 0.64 +/- 0.22 ng/ml, respectively. Wide inter-individual variation was noted. The nitroglycerin-induced increase in the orthostatic rise in heart rate and the change in digital-pulse-wave morphology roughly paralleled the plasma concentration, whereas the reduction in systolic blood pressure in the upright position was still evident 15 mins after the infusion, i.e. when nitroglycerin was no longer measurable in plasma. No significant diurnal variation in vascular sensitivity to the vasodilative action of nitroglycerin was demonstrable. The change in pulse-wave morphology resulting from the reduction in peripheral resistance (shift of the dicrotic wave in the descending limb towards the base-line) proved to be the most sensitive haemodynamic parameter.