The whole-cell patch-clamp method was used to study the voltage-gated K+ conductance of human peripheral blood T lymphocytes. After entry into whole-cell recording mode, there are time-dependent changes in some properties of the conductance. Over the first 10-30 min, the threshold for activation shifts about 10 mV more negative, and the rates of activation and inactivation increase. Inactivation is less strongly voltage dependent than activation or deactivation. Lymphocytes were stimulated to proliferate in culture with the tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). No changes in K+ conductance were observed in the first few hours of TPA stimulation. At 24 h after mitogen addition, TPA-treated cells were found to have 1.7-fold greater average voltage-gated K+ conductance than unstimulated control cells. At 48 h, TPA-stimulated cells had the same average K+ conductance as at 24 h, even though the cells were now much increased in size, as measured by cell capacitance. DNA synthesis by cultures stimulated with TPA, phytohaemagglutinin or succinyl concanavalin A was depressed by the addition of 0.1 mM-quinine at any point in the culture period. In the first 20 h after mitogen addition, DNA synthesis was more effectively inhibited by quinine than if the drug were added later. Cell proliferation was equally sensitive to quinine regardless of mitogen.