In the present study, we investigated time course changes of water status including relative water content (RWC), leaf osmotic potential (Psi(Pi)), stomatal conductance (g,), proline (Pro), chlorophyll fluorescence (F-v/F-m) and total chlorophyll content in the Arabidopsis thaliana under PEG-induced drought stress after exogenous ABA treatment. To a better explanation for the role of ABA in the water status of A. thaliana to drought stress, wild-type (Columbia) and ABA-deficient mutant (aba2) of A. thaliana were used in the present study. Moreover, three weeks old Arabidopsis seedlings were applied exogenously with 50 p,M ABA and exposed to drought stress induced by 40% PEG8000 (-0.73 MPa) for 6 h, 12 h and 24 h (hours). Our findings indicate that RWC of wild-type and aba2 started to decrease in the first 12 h and 6 h of PEG-induced drought stress, respectively. However, exogenous treatment of 50 mu M ABA increased their RWC under drought stress. On the other hand, while We of both genotypes started to decrease in the first 6 h of drought stress, these declines in We were prevented by ABA treatment under stress throughout the experiment; it was more pronounced in aba2 at 24h. While the highest increase in gs was obtained in aba2 after 24 h stress, ABA-induced highest decrease in g5 was obtained in the same genotype during 12 h, as compared to PEG-treated group alone. On the other hand, Pro content increased in all treatment groups of ABA-deficient mutant aba2 at 12 h and 24 h. However, Pro content in ABA+ PEG treated aba2 plants was higher than in PEG- and ABA-treated plants alone at the end of the 24 h. Drought stress decreased Fv/Fn, and total chlorophyll contents of both genotypes while 50 1.1,M ABA alleviated these reductions during drought stress, as compared to PEG stressed plants. On the other hand, 50 LIM ABA treatment alone did not create any remarkable effect on F-v/F-m, and total chlorophyll contents.