The effect of uniform compression is studied on the superconducting transition temperature of the HTS ceramicsYBa2(Cu1−xAgx)4O8±&dgr;(x=0, 0.05, 0.1, 0.15).It is shown that the pressure effect in these compounds differs considerably from that in the systemYBa2(Cu1−xAgx)3O7−&dgr;.The observed peculiarities are discussed on the basis of the van Hove anomaly in the electron energy spectrum of oxide cuprates. The correspondence of the superconducting properties of the 1–2–4 and 1–2–3 systems under pressure with certain models of high-temperature superconductivity is discussed. The nature of nonlinearity of the temperature dependence of the electrical resistance of the 1–2–4 system in the normal state and the effect of substitution of silver for copper on it are studied. The corresponding phase diagram is constructed. The correspondence of the observed increase in crossover temperature with the Nagaoza–Lee theory is discussed. ©1997 American Institute of Physics.