Responses to ionophoretically-applied noradrenaline were investigated with micro-electrodes in whole tissue preparations and with patch pipettes in isolated cells dispersed from the guinea-pig pulmonary artery. In whole tissue and dispersed cells noradrenaline evoked monophasic depolarizations which had a similar time course. In dispersed cells the amplitude of electronic potentials was reduced during the noradrenaline-evoked depolarization. Under voltage clamp noradrenaline elicited an inward current, which persisted in 18 mM external potassium with the membrane potential set at the potassium equilibrium potential. In voltage clamp experiments the amplitude of current steps to hyperpolarizing voltage jumps was increased during the noradrenaline-induced inward current. These data suggest that the depolarization to noradrenaline in the guinea-pig pulmonary artery is mediated by an increase in membrane conductance.