The efficacy of evoked ACh release by intact and newly sprouted terminals in response to partial denervation and expansion of the motor neuron terminal field was studied in mouse soleus muscle after section of the L-5 spinal root. From 2 to 4 d after partial denervation until 90 d later, only 3-7 motor units of the normal 21 remained. Regeneration of the dissected nerve was prevented while the remaining motor units were sprouting. The indirect twitch, which was only 20% of direct twitch tension 2-4 d after nerve section, recovered between 28 and 50 d postoperatively. However, the depression of twitch in low Ca/high Mg solution, which was equal to control 2-3 d postoperatively, was 2-3 times more depressed than control by 50 d and remained so up to 90 d. This indicated persistent reduction of the safety factor in sprouted motor units. Intracellular measurement of quantal content in 0.4 mM Ca, 2.75 mM Mg revealed 2 groups of nerve terminals in partially denervated muscle. The quantal content of the first group was greater than contralateral control at earlier times (28-50 d) and only slightly greater than control later (74 and 90 d). This group consisted of the original undenervated terminals, since it was associated with normal miniature endplate potential (MEPP) frequency and end-plate potential (EPP) latency, and presumably with the class of fibers with normal (zinc iodide osmium-stained) nerve terminal morphology and occasional large myelinated preterminal axons.(ABSTRACT TRUNCATED AT 250 WORDS)