A dose response relationship and the time of onset to 50 per cent and 100 per cent peak action were investigated for neostigmine reversal of atracurium in the dog. Two levels of neuromuscular block were used, 10 per cent and 50 per cent of the first twitch of the train of four. The ED50 from the first group was 0.1 mg kg-1 and for the second group was 0.019 mg kg-1. There was little difference between the onset times at the two levels of block. It is concluded that the main factor in determining the dose of neostigmine is the depth of the initial blockade.