Upper and lower bounds on first-order rate-distortion functions are derived for some selected speech probability models and two different fidelity criteria. The bounds are computed numerically when the analytical procedures are intractable. The models employed involve the Laplacian density, the gamma density, and first- and second-order modified Bessel functionsK_{o}andK_{1}. For both the absolute difference and the mean-square difference distortion measures, theK_{o}model yields the tightest bounds onR_{1}(D)functions.