1. Six sheep, three animals per diet, were prepared with rumen fistulas and fed on frozen grass or grass-maize pellets to give magnesium intakes of 1.79 and 2.23 g/d respectively. The mean apparent availabilities of Mg in sheep fed on frozen grass and grass-maize pellets were 0.31 and 0.36 respectively. 2. The rumen contents were fractionated by straining the digesta through linen cloth and then differentially centrifuged to give 20,000 g and 100,000 g supernatant fractions. 3. In all sheep, regardless of diet, at 4 and 16 h after a meal, 50 and 60% respectively of the total Mg in the rumen contents was found in strained rumen fluid while 30 and 38% respectively of the total Mg was found in the 100,000 g supernatant fraction. 4. The net absorption of Mg from the temporarily isolated and washed reticulo-rumen was studied using either 100,000 g supernatant fractions of rumen contents from sheep fed on one or other of the two diets, or inorganic buffers containing the same concentration of Mg and other macroelements. 5. The Mg was readily absorbed from the 100,000 g supernatant fraction placed in the rumen with the rate of absorption being 7.3 mumol/l per min (505 mg/d) from the supernatant fraction obtained from sheep fed on frozen grass and 11.3 mumol/l per min (781 mg/d) from the supernatant fraction from sheep fed on grass-maize pellets. In the same sheep, the previously described rates of Mg absorption from the 100,000 g supernatant fraction were similar to those obtained from the comparable inorganic buffers. 6. The effects of varying concentrations of potassium and sodium on the net absorption rate of Mg (as 24Mg) and on the one-way efflux of Mg (as 28Mg) from supernatant fractions or rumen fluid and inorganic buffers were investigated using the temporarily-isolated and washed rumen in three sheep. Although the net absorption rate of 24Mg from supernatant fractions or buffers containing similar K concentrations varied significantly between sheep, a similar percentage decrease in the absorption rates of both 24Mg and 28Mg was found for each sheep as the K concentration was increased. 7. One pair of sheep was fed on the frozen grass and the other pair was fed on the grass-maize pellets. Their daily intakes of K were then increased to 50 g/d for 14 d by intrarumen infusion of potassium chloride. In three of the four sheep the plasma Mg concentration fell within 12 h of the start of the KCl administration.(ABSTRACT TRUNCATED AT 400 WORDS)