The effect of a magnetite Fe3O4 nanopowder in a nanocomposite with manganite La0. 67Sr0. 33MnO3 on its magnetoresistive effect and conductivity is studied. The addition of a magnetite nanopowder to the initial material leads to a substantial (twofold) increase in the low-field magnetoresistive effect. The low-field magnetoresistive effect in substituted lanthanum manganite nanopowders is based on spin-dependent electron tunneling through grain boundaries. The transport mechanisms in such systems are analyzed, and noise spectroscopy is also used for this purpose.