Frequency and dose-dependent changes in neuromuscular transmission were examined in 70 patients undergoing elective surgical procedures requiring the use of muscle relaxants. Anesthesia was induced with sodium thiopental and maintained with N2O-O2 and fractional dose of meperidine or fentanyl. Neuromuscular block was produced and maintained at 80% level with incremental intravenous doses of d-tubocurarine. Neuromuscular function was measured by recording the force of thumb adduction evoked by supramaximal stimulation of the ulnar nerve at the wrist. Single stimuli were applied every 2.5 seconds as square pulses of 0.1-msec duration. In 30 patients tetanic trains of 10-second duration ranging from 10 to 400 Hz were used. In 40 patients neostigmine doses ranging from 0.5 to 5.0 mg were used. Criteria for responses to 10-second tetanic trains during 80% neuromuscular block were determined and compared with responses after neostigmine. At a frequency of 50 Hz a complete tetanic fade was followed by partial posttetanic relief of block. At a frequency of 200 Hz tetanic fade was followed by complete but transient posttetanic decurarization. The original control twitch tension was not exceeded in posttetanic or postdrug responses. It is concluded that the transient after effects of tetanic stimulation are closely related to the anticurare effects of neostigmine.