There is an increased accumulation of message for the catalytic (70-kDa) subunit of the tonoplast H +-ATPase in leaves of tomato (Lycopersicon esculentum L.) plants responding to NaC1. To determine if abscisic acid (ABA) mediates this response, message accumulation was examined in treatments designed to separate exposure to NaC1 from increases in endogenous ABA. Under three different experimental conditions, salt-induced changes in the accumulation of 70-kDa message were unrelated to any change in endogenous ABA. The results were as follows: (i) under drought stress, plants accumulated levels of ABA similar to those measured in salt-treated plants; however, no increase in 70-kDa subunit message was observed; (ii) the ABA-deficient mutant sitiens exhibited an increased accumulation of message despite the absence of NaCl-induced accumulation of ABA; and (iii) the inhibitor of general isoprenoid biosynthesis, Lovastatin, blocked NaCl-induced accumulation of ABA but did not alter NaCl-induced accumulation of message. In addition to these three experimental responses, application of exogenous ABA increased endogenous ABA levels without any comparable increase in message accumulation. Based on these results, it is concluded that ABA does not mediate the NaCl-induced accumulation of 70-kDa subunit tonoplast H+-ATPase message accumulation in tomato.