Uncertainties from the literature concerning the role of apolipoprotein E (apoE) in central cholinergic function prompted us to investigate what effect apoE may have on transmission at the neuromuscular junction. Both spontaneous and evoked release were measured in isolated extensor digitorum longus (edl) and soleus muscles from both wild-type and apoE-deficient mice. Miniature endplate and nerve-evoked endplate potentials (MEPPs and EPPs, respectively) were indistinguishable in edl muscles in both groups of mice; however, MEPP amplitudes in soleus muscles were significantly larger (by an average of 23%) in apoE-deficient mice compared with 5- to 7-week-old age-matched wild-type mice. The EPP amplitudes were also larger in soleus muscles in the mutant mice, but this was a reflection of the larger quantal size in this muscle because quantal content, determined from the ratio of the average EPP amplitude to average MEPP amplitude, was unchanged from normal in the mutant mice. The MEPP frequency and the percent of nerve stimulations failing to produce an EPP were unchanged from normal in both muscle types in the mutant mice. The difference in quantal size in soleus muscle transmission between mutant and wild-type mice was abolished in the presence of neostigmine, an acetylcholinesterase inhibitor. The results suggest that apoE normally associates with acetylcholinesterase in the synaptic cleft of slow muscles, modulating the activity of the enzyme and therefore quantal size.