Osmotic substrates, comprising 5-and 10-atm. concentrations of KNO3, NaCl, mannitol, and sucrose were added to standard culture solutions to determine the effect on relative turgidity, DPD, osmotic potential, transpiration, and growth of tomato plants. Cl4-labelled mannitol and Cl36-labelled sodium chloride were incorporated into the high-concentration treatments to provide additional information on solute absorption. After an initial loss of water content (fresh weight-dry weight) and associated wilting, recovery took place rapidly in all except the mannitol treatments and after 28 hours did not differ significantly from the control. At this time the 1st stage of the experiment was concluded and all plants were replaced in standard culture solutions. Recovery of water content and turgor during the 1st stage was associated with a rapid increase in internal osmotic potential. This proceeded to such an extent that, by the end of stage 1, the internal potential in each treatment exceeded the substrate potential by approximately the same amount as the potential of the control exceeded that of the base nutrient solution. The increase in internal osmotic potential was achieved mainly by solute absorption except in the mannitol treatments in which dehydration appeared to account for almost half of the increase. Absorption of Cl36 from labelled NaCl was closely related to the increase in osmotic potential in the high-concentration NaCl treatment, the relative concentration of the isotope in the plant at the end of stage 1 compared with concentration in the substrate being approximately 11. In the mannitol treatment the relative concentration of C14 was only 1:4, confirming that mannitol absorption alone could only account for a small proportion of the total increase. Following the removal of the osmotic substrates an initially rapid and progressive decline in internal osmotic potential occurred, together with an associated reduction in Cl36 and Cl4 concentration in the tops of the plant. In both cases this decline could be satisfactorily accounted for by increased water content of the plants, leakage of solutes to the substrate being negligible. The effect of the osmotic treatments on transpiration appeared to be closely related to changes in volume (expressed as water content), showing an initial severe reduction and subsequent recovery to values close to the control in all except the high-concentration mannitol treatment. Because of the short term nature of the experiment no significant changes in growth (measured as increase in dry weight) could be detected in stage 1. Following the removal of the osmotic substrates however, dry weight increase in the low-concentration sucrose and potassium nitrate treatments was more rapid than in the control and in the high-concentration mannitol treatment was significantly less.