Double mixing experiments using a three-syringe stopped-flow apparatus have given values of the second-order rate constants for association of epsilon ATP, ATP, and ADP to G-actin of 6.8 X 10(6) M-1 s-1, 6.1 X 10(6) M-1 s-1, and 6.3 X 10(6) M-1 s-1, respectively, at pH 7.6, 20 degrees C, and 0.65 mM free Ca2+. The previously established ca. 100-fold weaker binding of ADP than ATP to G-actin is due to a much faster dissociation rate of ADP than ATP, rather than to a slower association rate as was previously reported. This difference between ADP and ATP largely disappears under more nearly physiological conditions (0.8 mM Mg2+ and 100 mM KCl). Association rate constants for the three nucleotides under these conditions are 2.13 X 10(6) M-1 s-1, 1.1 X 10(6) M-1 s-1, and 1.2 X 10(6) M-1 s-1, respectively, for epsilon ATP, ATP, and ADP. The rate constant for association of epsilon ATP is only slightly affected by reducing the Mg2+ concentration from 0.8 to 0.2 mM, whereas that for ADP association is reduced by a factor of ca. 3. This, together with the observed increase in the apparent association rate constant of epsilon ATP on increasing the Ca2+ concentration in the 10-100 microM range, suggests that G-actin recognizes and binds the metal-nucleotide complexes.