The experimental results of isothermal pressure dielectric permittivity epsilon studies in a critical mixture characterized by a negative shift of critical temperature induced by pressure (dT(C)/dP<0) are presented. The critical effect is portrayed by the same relation as in previous epsilon(T) and epsilon(P) studies, with the critical exponent alpha=0.12+/-0.03. The advantage of pressure studies is the negligible influence of the correction-to-scaling term and the low-frequency Maxwell-Wagner effect. This conclusion is supported by the distortion-sensitive derivative analysis of the experimental data. In contrast to previous epsilon(P) studies, carried out in mixtures with dT(C)/dP>0, the critical effect manifests by the bending-up behavior near the critical point. It is suggested that signs of the critical amplitudes of epsilon(P) and epsilon(T) anomalies may be related to the excess volume V(E) and the excess enthalpy H(E), respectively.