Five crossbred heifers were slaughtered at different times and rumen and omasum tissues were removed: The epithelium was separated from the muscle layer and mounted in parabiotic chambers. The tissues were incubated for 2 h in buffer at 39 C and aerated continuously in 95% O₂ 5% CO₂ . The buffers were similar to those which have been used previously in mineral transport studies, but were modified by varying the levels of Mg and K while keeping the osmolarity constant. Potassium was included in the buffers on the mucosal side in appropriate ratios to Mg to correspond to the K: Mg in diets fed to ruminants which would contain .1% Mg and .6, 2.4 and 4.8% K, (low, medium and high, respectively). A fourth buffer which contained physiological concentrations of Kand Mg, was placed on the serosal side of the parabiotic chamber. Flow of Mg was calculated by: F = C₁V₁ - C₀V₀ / AΔt where C₁ = final concentration, C₀ = original concentration, V₁ = final volume, V₀ = original volume, Δt = incubation time, A = area of the tissue exposed to the buffer, and F = flow of Mg (mg/cm²/h). Blanks were included which contained physiological levels of Mg on both side of the chamber to adjust for tissue effects. Magnesium transport tended to be 10 times greater through the rumen than the omasum. This indicates that the rumen is the primary site of Mg absorption in cattle. Potassium tended to depress Mg transport across both tissues. This technique has only limited application in mineral research. / M.S.