The method of parent compounds has been used to establish an approximate correlation between the variation of the critical temperature, {ital T}{sub {ital c}}, and the variation of the potential difference, {Delta}{ital V}{sub {ital A}}, of a hole at the apex and planar oxygen sites within the superconducting phase region of YBa{sub 2}Cu{sub 3}O{sub {ital x}}. The resulting model was used to predict the pressure derivative of {ital T}{sub {ital c}} under hydrostatic pressure, giving derivatives and standard uncertainties of {ital dT}{sub {ital c}}/{ital dP}=6.1{plus_minus}0.5 K/GPa for {ital x}=6.6 and {ital dT}{sub {ital c}}/{ital dP}={minus}0.6{plus_minus}0.3 K/GPa for {ital x}=6.93. {copyright} {ital 1996 The American Physical Society.}