An experimental study of proximity effect in La_0.67Sr_0.33MnO₃–YBa₂Cu₃O₇–La_0.67Sr_0.33MnO₃ trilayers is reported. Transport measurements on these samples show clear oscillations in critical current ($I_{c}$) as the thickness of La_0.67Sr_0.33MnO₃ layers ($d_{F}$) is scanned from $\sim 50$ Å to $\sim 1100$ Å. In the light of existing theories of ferromagnet–superconductor (FM–SC) heterostructures, this observation suggests a long range proximity effect in the manganite, modulated by its weak exchange energy ($\sim 2$ meV). The observed modulation of the magnetic coupling between the ferromagnetic LSMO layers as a function of $d_{F}$, also suggests an oscillatory behavior of the SC order parameter near the FM–SC interface.