1. Frog sartorius muscles were treated with an irreversible cholinesterase inhibitor and then incubated in Ringer with 2 mM-LaCl3. The amounts of ACh in the tissue and medium were assayed by mass fragmentography, miniature end-plate potentials (min. e.p.p.s) were recorded and the end-plate was investigated by electron microscopy. 2. Addition of La3+ caused in normal, but not in denervated, muscles a discharge of both min. e.p.p.s and chemically detectable ACh. After 30 min both min. e.p.p.s and ACh release decreased. Between 4 and 5 hr after the addition of La3+ min. e.p.p.s had practically ceased and the rate of ACh release was almost back to that in the absence of La3+. 3. La3+ caused a 50% reduction in the ACh content of the tissue within the first 30 min; thereafter ACh gradually increased to 110% by 5 hr. At this time synaptic vesicles were practically absent in most terminals. The ACh was predominantly located in the end-plate regions of the muscles, before as well as after the incubation with La3+. ACh in end-plate free parts of the muscles was unchanged by La3+. 4. Hemicholinium-3 inhibited the synthesis of ACh in the muscles, but it had almost no influence on La3+-induced ACh release. 5. From these and other results, it is concluded that the ACh released by La3+ originates exclusively from the nerve terminals, that most likely this ACh is released via exocytosis from synaptic vesicles, and that the synthesis of ACh following the release of ACh takes place in the nerve terminals. The results further indicate that in freshly excised muscle the greater part (80-90%) of the ACh contained in the nerve terminals is located in the vesicles.