The effects of hypersaline treatment (osmotic upshock) on cell water relations were examined in the Gram-positive bacterium Bacillus subtilis using particle size analysis. Application of the Boyle-van't Hoff relationship (cell volume versus reciprocal of external osmolality) permitted direct determination of turgor pressure, which was approximately 0.75 osmol kg-1 (1.9 MPa) in exponentially growing bacteria in a defined medium. The abolition of turgor pressure immediately after upshock and the subsequent recovery of turgor were investigated. Recovery of turgor was K+ dependent. Calculation of turgor by an alternative method involving spectrophotometric analysis of shrinkage gave somewhat lower estimates of turgor pressure.