High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and (11 (2) over bar0) nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (E-PL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of E-PL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of E-PL, dE(PL)/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dE(PL)/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dE(PL)/dp with the QW width is due to the pressure-induced increase in the built-in electric field F-int. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the F-int increases with pressure with a rate of about 80 kV(cmGPa)(-1). (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3043888]