A number of experiments was carried out to determine the sequence of events leading to death following administration of the euthanizing agent T61. Simultaneous recordings of the EMG, EEG, ECG and end-tidal CO2 (dogs only) were obtained in acutely instrumented rabbits and dogs. Results show that following T61 administration the loss of consciousness and loss of muscle activity occurred simultaneously. Vocalization and increased muscle movement occurred in the initial phase of the injection in 3 of 8 dogs, injected with T61 or butyramide. From this study it was concluded that the presence of the muscle relaxant does not pose an ethical problem for the use of T61 as an euthanizing agent, but our results suggest that the use of T61 may have some emotionally unpleasant side-effects.