 The 4077th actually consisted of two separate sets. An outdoor set,  located in the mountains near Malibu, California, was used for all  exterior and tent scenes for the first few seasons. The indoor set,  located on a sound stage at Fox studios, was used for the indoor  scenes for the run of the series. Later, after the indoor set was  renovated to permit many of the "outdoor" scenes to be filmed there,  both sets were used for exterior shooting as script requirements  dictated (for example, night scenes were far easier to film on the  sound stage, but scenes at the chopper pad required using the  ranch).