Seizure thresholds were measured in patients treated with right unilateral electroconvulsive therapy (ECT) using 0.9 amp, 0.5 ms constant current pulses at 30 Hz for 0.93-2.8 s. Using these parameters, only two patients failed to seize at stimulus charges < or = 75.6 mC. Three patients were excluded from further analysis because they experienced generalized motor or electrographic seizures lasting less than 25 s and 30 s, respectively. The remaining 52 patients had a mean seizure threshold of 48.9 +/- 15.3 mC. Seizure threshold was correlated with age, but not sex. These values for seizure threshold are significantly lower than those reported previously using longer pulse-briefer train duration stimuli, suggesting that characteristics of the electrical stimulus influence seizure threshold. These findings have implications for electrical dosing in right unilateral ECT.