The distribution and turnover rate of acetylcholine receptors labeled with 125I-alpha-bungarotoxin were examined in innervated mouse sternomastoid muscle by electron microscope autoradiography using the "mask" analysis procedure. We compared the total population of receptors with receptors newly inserted at the junction 2 d after inactivation with nonradioactive alpha-bungarotoxin, both at the top (thickened) region of the postjunctional folds (pjm) and the nonthickened bottom folds. We found that the receptor site density was approximately 10 times greater on the thickened pjm than on the nonthickened bottom folds for both total and newly inserted receptors. This ratio does not change significantly during a 6-d period after labeling the new receptors. Furthermore, calculated values for turnover time of receptors show that both total and newly inserted receptors at both regions of the junctional folds have half-lives for degradation within the range given in the literature for slow junctional receptors. These data exclude a simple migration model whereby receptors are preferentially inserted in the nonthickened region of the junctional folds and then migrate into the thickened membrane at a rate equal to the turnover rate of the receptors.