The entry of labelled calcium into the innervated region of the mouse diaphragm muscle.

1. A method is described for measuring the distribution of (45)Ca between the junctional and non-junctional regions of the mouse diaphragm muscle.2. Muscles which were incubated in Ringer solution containing labelled calcium accumulated the tracer at the junctional region when acetylcholine or carbachol was present. Denervation did not prevent this accumulation at the normally innervated region.3. Histochemical evidence indicated that, in the presence of acetylcholine or carbachol, calcium entered diaphragm muscle fibres at the junctional region.4. Accumulation of calcium occurred at all regions of muscle fibres depolarized by potassium. In the presence of acetylcholine or carbachol, accumulation occurred at the junctional region in the absence of muscle fibre post-junctional membrane depolarization.5. It is suggested that the desensitized muscle fibre post-junctional membrane has an increased calcium permeability.