When incubated in 150 mM KCl, rat thymocytes exhibited a very important magnesium efflux (11.4 +/- 0.7 mmoles/liter cells/20 min, n = 29), about 90 times higher than the physiological magnesium efflux catalyzed by the Na-Mg exchanger (0.126 +/- 0.093 mmoles/liter cells/20 min), Cells remained viable (trypan blue test) and membrane integrity was shown by the absence of an increase in sodium permeability, K+-induced magnesium efflux exhibited the following properties: (i) it required the presence of external chloride; (ii) it was fully blocked by DIOA, a selective KCl-cotransporter inhibitor (IC50 = 35 mu m); and (iii) it was associated to a progressive increase in cell volume via the DIOA-sensitive K-Cl cotransporter, Such cell swelling seems to play a causal role, because (i) hypertonic media (+400 mM sucrose) abolished K+-induced magnesium efflux and (ii) hypotonic Ringer media (205 mOsm) increased both cell volume and magnesium efflux (from a basal value of 0.35 +/- 0.03 mmoles/liter cells/20 min up to 1.44 +/- 0.24 mmoles/liter cells/20 min), even in the presence of DIOA. In conclusion, high potassium induced a dramatic release of intracellular magnesium from rat thymocytes, Such a phenomenon was, at least in part, caused by cell swelling via the DIOA-sensitive K-Cl cotransporter. The nature of the magnesium transport mechanism and its role in the transduction signal of K-Cl cotransporter activation by cell swelling deserve further investigation.