1. Dose-response curves of alcuronium, fazadinium, gallamine, metocurine (dimethltubocurarine), pancuronium, and tubocurarine were obtained from 362 surgical patients during general anaesthesia (barbiturate, fentanyl, droperidol, N2O; normoventilation) by means of mechanomyograms of the hand muscles after supramaximal electrical stimulation (100-125 V, 0.2 ms, 0.2 Hz) of the ulnar nerve. - 2. In a conventional semilogarithmic plot of effect E/Emax versus dose D the sigmoid-shaped curves exhibit a parallel shift with almost the same slopes. - 3. A linear plot of E/Emax against D yields curves intersecting by extrapolation with the ordinate at a point s below zero which indicates a threshold phenomenon in the dose-response relation and gives the value for this threshold (s=60-80). - 4. A double-reciprocal plot of 1/(E/Emax) versus 1/D does not result in straight lines, indicating a lack of linear proportionality between receptor occupancy and effect. The curves are straightened plotting 1/[(E+S)/(Emax+S)] versus 1/D. - 5. From our results which are supported by the "margin of safety" concept of neuromuscular transmission, it is supposed that the relation between neuromuscular blocking drugs and nicotinic receptors does not follow the classical occupation theory based on the law of mass action.