1. Mannitol hexanitrate (MANHN), iditol hexanitrate and sorbitan tetranitrate, all at 442 muM, stimulate oxygen uptake by isolated rabbit atrial tissue.2. At the same concentration, glyceryl trinitrate decreased oxygen uptake, while isosorbide dinitrate and erythrityl tetranitrate had no effect.3. No simple correlation was found between the lipoid solubility or per cent nitration of these compounds and their effects on oxygen uptake.4. The stimulating effect of MANHN and its possible role as an uncoupler of oxidative phosphorylation is discussed.