In isoalted human pulmonary arteries and veins the contractile response to noradrenaline (1.8 X 10(-5)M) was 33 +/- 7.4% and 20 +/- 4.5% (Mean +/- S.E.M.) of that induced by potassium (127 mM). A variable degree of spontaneous contractile activity was recorded in the vein preparations. This activity was abolished by nifedipine (2.9 X 10(-6)M). Digoxin (10(-6)M) induced contractions in pulmonary vessels. In the arteries, the digoxin contraction developed slowly and reached a maximum amplitude of 90 +/- 4% (Mean +/- S.E.M.) of the potassium evoked contraction. In the veins, the amplitude of the digoxin contraction was 32 +/- 5% of that induced by potassium. Digoxin (10(-6)M) also increased the maximum response to noradrenaline and potassium in both arteries and veins. In the arteries, the noradrenaline and potassium contractions increased to 211 +/- 6.8% and 145 +/- 8.9 of control, and in the veins to 169 +/- 13.5% and 163 +/- 9.9%, respectively. Nifedipine in concentrations which completely relaxes arterial and venous preparations contracted by potassium, had only a slight relaxing effect on digoxin induced contraction. It is concluded that digoxin increases the tension in pulmonary arteries and veins, and increases the maximum response to noradrenaline and potassium in both types of vessels. The digoxin induced contraction is highly resistant to blockade of extracellular calcium influx.