Given a dynamical system associated to a rational function ϕ(T ) on P 1 of degree at least 2 with coefficients in a number field k, we show that for each place v of k, there is a unique probability measure µ ϕ,v on the Berkovich space P 1 Berk,v /C v such that if {z n } is a sequence of points in P 1 (k) whose ϕ-canonical heights tend to zero, then the z n 's and their Galois conjugates are equidistributed with respect to µ ϕ,v . In the archimedean case, µ ϕ,v coincides with the well-known canonical measure associated to ϕ. This theorem generalizes a result of Baker-Hsia [BH] when ϕ(z) is a polynomial.The proof uses a polynomial lift F (x, y) = (F 1 (x, y), F 2 (x, y)) of ϕ to construct a twovariable Arakelov-Green's function g ϕ,v (x, y) for each v. The measure µ ϕ,v is obtained by taking the Berkovich space Laplacian of g ϕ,v (x, y), using a theory developed in [RB]. The other ingredients in the proof are (i) a potential-theoretic energy minimization principle which says that g ϕ,v (x, y) dν(x)dν(y) is uniquely minimized over all probability measures ν on P 1 Berk,v when ν = µ ϕ,v , and (ii) a formula for homogeneous transfinite diameter of the v-adic filled Julia set K F,v ⊂ C 2 v in terms of the resultant Res(F ) of F 1 and F 2 . The resultant formula, which generalizes a formula of DeMarco [DeM], is proved using results from [RLV] about Chinburg's sectional capacity. A consequence of the resultant formula is that the product of the homogeneous transfinite diameters over all places is 1.