Both types of vesicles were loaded for 120 min at ambient temperature in a medium consisting of 100 mM mannitol, 3 mM EGTA, 60 mM NMG-nitrate, 40 mM NMG-gluconate, 125 mM Tris-Mes, pH 6.6, and with either 0 or 2.5 mM ammonium acetate. Osmolarity was maintained constant with sucrose. Membrane vesicles were diluted 1 : 41 in a NH4+-free medium consisting of 0.5 mM 22Na+, 100 mM mannitol, 3 mM EGTA, 60 mM NMG-nitrate, 40 mM NMG-gluconate, and 125 mM Tris-Mes, pH 6.6, and incubated for 9 s. DMA (0.5 mM) was added to the incubation medium where indicated. Values are means ±s.e.m. of nine determinations on three different BLMV and LMV preparations. *P < 0.01for 2.5 mM (NH4+)ivs. 0 mM (NH4+)i; and for 2.5 mM (NH4+)i+ DMA vs. 2.5 mM (NH4+)i, by ANOVA.