This study investigates the effects of heterogeneities on the estimates of effective or relative permeabilities from well-test pressure data obtained from a well producing a solution-gas-drive reservoir. The heterogeneous system considered in this work is that of a two-zone composite reservoir with different absolute permeability and/or relative permeability curves in the two regions. The estimation of effective (or relative) permeability values as a function of sandface oil saturation or wellbore pressure for both drawdown and buildup tests is presented. It is shown theoretically and computationally that for the two-zone problem of this work, one usually can obtain only one of the outer zone effective (or relative) permeabilities (either oil or gas) evaluated at the sandface value of oil saturation. It is also shown that the effective permeabilities estimated from pressure drawdown or buildup data reflect, at early times, the inner zone effective permeability data, and at later times reflect the outer zone effective permeability data for the oil phase. If the initial reservoir pressure is above the reservoir bubble-point pressure then the effective oil permeability of the inner or the outer zone might be obtained from the analysis of drawdown and/or buildup pressure data.