This paper reports experiments carried out between 1.5 and 2.3 GPa using synthetic analogs of the molten peridotite system to investigate the effects of pressure (P), temperature (T), and variable bulk chemistry on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and spinel (garnet). The multidimensional surface defined by melts coexisting with the lherzolite mineral assemblage produced in this study as well as from the literature is fit as a function of the variables: P, melt Mg number, and melt mole percent NaO0.5, KO0.5, Cr2O3, and TiO2, using multiple regression techniques. Forward models of polybaric, near-fractional melting using the parameterization presented here demonstrate that small extent, pooled magmas produced at greater initial pressures of melting differ in composition from small extent, pooled magmas produced at lower initial pressures of melting. Furthermore, the small extent, pooled magmas generated at shallower initial pressures of melting fractionate at low P to magmas that are similar to the high-Na2O, low-FeO mid-ocean ridge basalts of the global array of Klein and Langmuir <1987>, while the small extent, pooled magmas generated at greater initial pressures of melting fractionate at low P to magmas that do not resemble any mid-ocean ridge basalts present in the global data set. Preliminary data relevant for melting garnet-lherzolite indicate that the systematics of the key major element indicators of depth of melting (FeO and SiO2 in the melt) observed in melts generated in the spinel stability field persist in melts generated in the lower P portion of the garnet stability field.¿ 1997 American Geophysical Union