To evaluate the role of mineralized tissues of the leopard frog in buffering acid, we analyzed the composition of femur and auditory capsule, the latter of which encloses a portion of the endolymphatic lime sacs, and investigated the extent to which these tissues are involved in buffering lactic acid after 2.5 h of anoxia and 10-19 min of strenuous exercise at 15 degrees C. We analyzed the following tissues for lactate: plasma, heart, liver, gastrocnemius muscle, femur, auditory capsule and carcass. Plasma [Ca(2+)], [Mg(2+)], [inorganic phosphate (P(i))], [Na(+)] and [K(+)] were also measured. Femur Ca(2+), P(i) and CO(3)(2-) compositions were similar to bone in other vertebrates. Auditory capsule had significantly more CaCO(3) than femur. Lactate was significantly elevated in all tissues after anoxia and exercise, including femur and auditory capsule. Anoxia increased plasma [Ca(2+)], [Mg(2+)], [P(i)] and [K(+)] and had no effect on plasma [Na(+)]. Exercise increased plasma [Mg(2+)], [P(i)] and [K(+)] and had no effect on plasma [Ca(2+)] or [Na(+)]. The skeleton and endolymphatic lime sacs buffered 21% of the total lactate load during anoxia, and 9% after exercise. The exact contribution of the entire endolymphatic sac system to lactate buffering could not be determined in the present study. We conclude that the mineralized tissues function as buffers during anoxia and exercised induced lactic acidosis in amphibians.