While ferromagnetism in CMR perovskites is satisfactorily explained by double exchange, its nature in CMR pyrochlore Tl2Mn2O7 is unclear. Here we investigate this problem by examining the pressure effect on the Curie temperature of pyrochlores A(2)Mn(2)O(7), A = Tl, In, Y, Lu. The data reveal that an applied pressure suppresses ferromagnetism. Such a behavior evidences against a currently favored simple FM superexchange picture that assumes an FM character of the 130 degrees Mn-O-Mn superexchange in Tl2Mn2O7. To account for experimental situation, we suggest that in the CMR pyrochlore nearest-neighbor superexchange couplings are antiferromagnetic, but frustrated, while ferromagnetism originates from the longer range interactions dominated by Mn3d-Mn3d superexchange via T16s orbitals. (C) 1999 Elsevier Science B.V. All rights reserved.