Verapamil, nifedipine and bepridil, three antagonists of L-type calcium channels in animal cells, were shown to induce direct inhibition of outward rectifying potassium current in Nicotiana tabacum cv. Xanthi protoplasts with IC50 of 5 microM, 5 microM and 1 microM, respectively. In the outside-out configuration, verapamil reduced the open probability of the ion channel responsible for the outward rectifying potassium conductance. Verapamil also blocked the outward rectifying potassium conductance in protoplasts from the N. tabacum cv. Bright Yellow cell line. Thus, studies using these molecules to demonstrate the involvement of calcium channels in plant physiological responses should be regarded with caution.