1. The binding of [3H]saxitoxin to innervated and denervated rat diaphragm muscle, and to normal frog muscle, has been measured. 2. A saturable component of saxitoxin binding, which was inhibited by tetrodotoxin, was detected in all preparations, as well as a component of non-saturable binding. The values for the maximum saturable capacity, M, and the equilibrium binding constant, K, for normal rat diaphragm muscle were: M = 24-4 f-mole.mg wet-1, and K = 3 -8 NM. 3. Denervation of rat diaphragm muscle reduced the maximum binding capacity per unit weight to 16-5 f-mole.mg-1. The value of K remained virtually unchanged at 4-2 nM. 4. It is suggested that the decrease in density per unit weight does not reflect any change in the density of sodium channels per unit area of membrane. 5. Two varieties of the same species of frog, Rana pipiens, were examined. In one variety (Southern) the value of M was 25-6 f-mole.mg-1 and the value of K was 4-3 nM. In the Northern variety the maximum binding capacity was less, M being 14-6 f-mole.mg-1; the value of K was 3-8 nM.