We have studied the postjunctional characteristics of motor endplates in the extensor digitorum longus (EDL) and soleus muscles of the rat. At voltage clamped endplates, equilibrium interactions between acetylcholine (ACh) and the ACh receptor were determined from the dose-response curves obtained by quantitative ionophoresis of ACh. These results showed that the maximum ACh induced conductance change per unit endplate surface, gmax, was 21.8 +/- 0.9 nS/microns2 in EDL and 8.2 +/- 0.9 nS/microns2 in soleus, the apparent dissociation constant, K, was 65.9 +/- 4.3 microM in EDL and 43.5 +/- 3.3 microM in soleus, and the Hill-coefficient, nH, was 2.3 +/- 0.1 in EDL and 2.2 +/- 0.1 in soleus. Single channel characteristics were derived from analysis of the ACh-induced endplate current noise. The results showed that at room temperature the mean conductance of the single channel, gamma, was 24.6 +/- 1.2 pS in EDL and 23.9 +/- 1.2 pS in soleus, and the mean life time of the channel, tau, was 0.80 +/- 0.05 ms in EDL and 0.71 +/- 0.03 ms in soleus. Of all the properties studied, the maximum conductance per unit endplate surface, gmax, was significantly smaller at the soleus endplate than at the EDL endplate. The calculated density of functional ACh receptors was 62% less, and the total number of the functional ACh receptors was 60% less at the soleus endplates than at the EDL endplates. These results suggest that the soleus has a lower margin of safety for neuromuscular transmission than the EDL.