T opical solvent treatment removes lipids from the stratum corneum leading to a marked increase in transepidermal water loss (TEWL). This disturbance stimulates a variety of metabolic changes in the epidermis leading to rapid repair of the barrier defect. Using an immersion system we explored the nature of the signal leading to barrier repair in intact mice. Initial experiments using hypotonic to hypertonic solutions showed that water transit per se was not the crucial signal. However, addition of calcium at concentrations as low as 0.01 mM inhibited barrier repair. Moreover, both verapamil and nifedipine, which block calcium transport into cells, prevented the calcium-induced inhibition of TEWL recovery. Additionally, trifluoroperazine or N-6-aminohexyl-5-chloro-1-naphthalenesulfonamide, which inhibit calmodulin, prevented the calcium-induced inhibition of TEWL recovery. Although these results suggest an important role for calcium in barrier homeostasis, calcium alone was only modestly effective in inhibiting TEWL recovery. Potassium alone (10 mM) and phosphate alone (5 mM) also produced a modest inhibition of barrier repair. Together, however, calcium and potassium produced a synergistic inhibition of barrier repair (control 50% recovery vs. calcium + potassium 0-11% recovery in 2.5 h). Furthermore, in addition to inhibiting TEWL recovery, calcium and potassium also prevented the characteristic increase in 3-hydroxy-3-glutaryl CoA reductase activity that occurs after barrier disruption. Finally, the return of lipids to the stratum corneum was also blocked by calcium and potassium. These results demonstrate that the repair of the epidermal permeability barrier after solvent disruption can be prevented by calcium, potassium, and phosphate. The repair process may be signalled by a decrease in the concentrations of these ions in the upper epidermis resulting from increased water flux leading to passive loss of these ions.