We study the relation between the Petersson norm of a holomorphic GL(2) form $f$ and that of its (suitably normalized) Jacquet-Langlands lift $g$ to a Shimura curve. The ratio of these two norms was previously shown to be algebraic by Shimura and rational by Harris-Kudla. We prove an integrality result for this ratio and explain the arithmetic significance of this ratio in terms of certain congruences satisfied by $f$