Department of Pure Mathematics, University of LeedsLeeds LS2 9JT, Abstract Harmonic maps from S 2 to S 2 are all weakly conformal, and so are represented by rational maps. This paper presents a study of the L 2 metric γ on M n , the space of degree n harmonic maps S 2 → S 2 , or equivalently, the space of rational maps of degree n. It is proved that γ is Kähler with respect to a certain natural complex structure on M n . The case n = 1 is considered in detail: explicit formulae for γ and its holomorphic sectional, Ricci and scalar curvatures are obtained, it is shown that the space has finite volume and diameter and codimension 2 boundary at infinity, and a certain class of Hamiltonian flows on M 1 is analyzed. It is proved that M n , the space of absolute degree n (an odd positive integer) harmonic maps RP 2 → RP 2 , is a totally geodesic Lagrangian submanifold of M n , and that for all n ≥ 3, M n is geodesically incomplete. Possible generalizations and the relevance of these results to theoretical physics are briefly discussed.