The kinetic properties of rabbit liver tRNA nucleotidyltransferase were examined. Our results indicate that incorporation of AMP into tRNA-C-C at pH 9.4 proceeds by a rapid equilibrium Random Bi Bi mechanism. This conclusion was supported by bisubstrate initial velocity studies, dead end and product inhibition, and isotope exchange at equilibrium and during the net reaction. This analysis has made possible calculation of the dissociation constants of various substrates and substrate analogs. The data indicate that binding of either a substrate or substrate analog greatly decreases the affinity of the enzyme for the second substrate. Comparison of binding constants for tRNAs revealed that affinities for the enzyme are tRNA-C-C greater than tRNA-C-Cp greater than tRNA-C-C-A. Also, ATP binds considerably more tightly than ADP. The apparent equilibrium constant of the forward reaction was determined and found to be about 1500 at pH 9.4, and about 50 at pH 7.0. Analysis of the reaction at pH 7.0 suggested that it proceeds by a similar mechanism, but that the dissociation of tRNA-C-C-A becomes more important. The relation of these in vitro properties of the enzyme to its cellular function are discussed.