The dependency of miniature endplate current (mepc) rise time upon mepc amplitude and acetylcholine receptor site density was measured in lizard intercostal muscles and used to fit the rate constants in a simple kinetic scheme. The kinetic scheme included diffusion, two-step sequential binding of acetylcholine to receptor, and opening of the ion channel. Numerical simulation of the observed mepc behavior yielded the following kinetic constants; (i) diffusion constant, 4 X 10(-6) cm2 sec-1; (ii) forward binding rates, 4.7 X 10(7) M-1 sec-1; (iii) channel relaxation rate, 25 msec-1. The value above for the forward binding rates assumed both rates to be equal. If they are different, the slower of the two is in the range of 2-5 X 10(7) M-1 sec-1. A radial profile of bound receptor indicated that activation of the receptor was very local, occurring essentially within a radius of about 0.3 micrometers from the point of acetylcholine release.