  One of the canons of condensed matter physics is the Onsager Reciprocity
principle in systems in which the Hamiltonian commutes with the time-reversal
operator. Recent results of measurements of the Nernst coefficient in
underdoped YBa_2Cu_30_{6+x}, together with the measurements of the anisotropy
of conductivity and the inferred anisotropy of the thermopower, imply that this
principle is violated. The probable violation and its temperature dependence
are shown to be consistent with the Loop-current phase which has been directly
observed in other experiments. The violation is related directly to the
magneto-electric symmetry of such a phase in which an applied electric field
generates an effective magnetic field at right angle to it and to the order
parameter vector, and vice versa.
