Compact extragalactic sources can display intensity fluctuations which are caused by fluctuations in the electron density of the interstellar medium. The 1/e time scale of these ‘‘refractive scintillations’’ can be many years. The intensity structure function D1(τ)=<[I—t)−I(T+τ)]2≳ has been computed for various distributions of the density fluctuations in the interstellar medium. The results show that the intensity fluctuations observed for periods much small than the 1/e time scale are dominated by the region near the earth. Thus most observations actually sample only the local region.