The working capacity of inbred female Swiss mice on the rotating rod was tested in an investigation of the effect of previously injected prednisolone, 1.2 mg, on the neuromuscular blocking effect of two different doses of d-tubocurarine and of low-dose hemicholinium-3. The effect of both drugs could be partially or wholly antagonized by prednisolone. The consequences of this finding are discussed in relation to the beneficial effect of prednisone in patients with myasthenia gravis.