This experimental study investigates the combined effect of the three primary Additive Manufacturing (AM) build orientations (0 degrees, 45 degrees, and 90 degrees) and an extensive array of heat treatment plans on the plastic anisotropy of maraging steel 300 (MS1) fabricated on the EOSINT M280 Direct Metal Laser Sintering (DMLS) system. The alloys microstructure, hardness, tensile properties and plastic strain behaviour have been examined for various strengthening heat-treatment plans to assess the influence of the time and temperature combinations on plastic anisotropy and mechanical properties (e.g. strength, ductility). A comprehensive visual representation of the material's overall mechanical properties, for all three AM build orientations, against the various heat treatment plans is offered through time - temperature contour maps. Considerable plastic anisotropy has been confirmed in the as-built condition, which can be reduced by aging heat-treatment, as verified in this study. However, it has identified that a degree of transverse strain anisotropy is likely to remain due to the AM alloys fabrication history, a finding that has not been previously reported in the literature. Moreover, the heat treatment plan (6h at 490 degrees C) recommended by the DMLS system manufacturer has been found not to be the optimal in terms of achieving high strength, hardness, ductility and low anisotropy for the MS1 material. With the use of the comprehensive experimental data collected and analysed in this study, and presented in the constructed contour maps, the alloys heat treatment parameters (time, temperature) can be tailored to meet the desired strength/ductility/anisotropy design requirements, either for research or part production purposes.