Presynaptic function was investigated at K+-stimulated motor nerve terminals in snake costocutaneous nerve muscle preparations exposed to carbonyl cyanide m-chlorophenylhydrazone (CCCP, 2 M), oligomycin (8 g x ml(-1)) or CCCP and oligomycin together. Miniature endplate currents (MEPCs) were recorded at -150 mV with two-electrode voltage clamp. With all three drug treatments, during stimulation by elevated K+ (35 mM), MEPC frequencies initially increased to values > 350 s(-1), but then declined. The decline occurred more rapidly in preparations treated with CCCP or CCCP and oligomycin together than in those treated with oligomycin alone. Staining with FM1-43 indicated that synaptic vesicle membrane endocytosis occurred at some CCCP- or oligomycin-treated nerve terminals after 120 or 180 min of K+ stimulation, respectively. The addition of glucose to stimulate production of ATP by glycolysis during sustained K+ stimulation attenuated the decline in MEPC frequency and increased the percentage of terminals stained by FM1-43 in preparations exposed to either CCCP or oligomycin. We propose that the decline in K+-stimulated quantal release in preparations treated with CCCP, oligomycin or CCCP and oligomycin together could result from a progressive elevation of intracellular calcium concentration ([Ca2+]i). For oligomycin-treated nerve terminals, a progressive elevation of [Ca2+]i could occur as the cytoplasmic ATP/ADP ratio decreases, causing energy-dependent Ca2+ buffering mechanisms to fail. The decline in MEPC frequency could occur more rapidly in preparations treated with CCCP or CCCP and oligomycin together because mitochondrial Ca2+ buffering and ATP production were both inhibited. Therefore, the proposed sustained elevation of [Ca2+]i could occur more rapidly.