In Vitis vinifera L. berries, the onset of ripening (known as "veraison") involves loss of turgor (P) in the mesocarp cells. We hypothesized that P loss was associated with an accumulation of apoplastic solutes in mesocarp tissue prior to veraison. Apoplastic sap was extracted from the mesocarp by centrifugation at the appropriate gravity to measure the apoplast solute potential (Psi (s) (A) ) and assay the sap composition. The Psi (s) (A) was about -0.2 MPa early in development, decreased about 1.0 MPa by veraison, and continued to decrease during ripening to almost -4.0 MPa by the end of berry development. Potassium, malate, tartrate, proline, glucose, fructose, and sucrose were quantified in apoplastic sap. The calculated contribution of these solutes was about 50% of the total Psi (s) (A) preveraison, but increased to about 75% as fructose and glucose accumulated during ripening. The contribution of the estimated matric potential to apoplast water potential decreased during development and was only 1.5% postveraison. We conclude that high concentrations of solutes accumulated in the mesocarp apoplast prior to veraison, and that P loss was a direct result of decreased Psi (s) (A) . Because Psi (s) (A) decreased before veraison, our findings suggest that apoplast solutes play an important role in the events of cellular metabolism that lead to the onset of ripening.