  We determine the density-dependent electron mass, m*, in two-dimensional (2D)
electron systems of GaAs/AlGaAs heterostructures by performing detailed
low-temperature Shubnikov deHaas measurements. Using very high quality
transistors with tunable electron densities we measure m* in single, high
mobility specimens over a wide range of r_s (6 to 0.8). Toward low-densities we
observe a rapid increase of m* by as much as 40%. For 2>r_s>0.8 the mass values
fall ~10% below the band mass of GaAs. Numerical calculations are in
qualitative agreement with our data but differ considerably in detail.
