We report saturated-absorption spectra recorded by use of 5 muW of infrared radiation coupled into a build-up cavity. Single-pass generation of difference-frequency radiation tunable near 4.25 mum in a periodically poled LiNbO(3) crystal was used. Lamb dips of weak transitions of the fundamental rovibrational band of CO(2) were observed, for what is believed to be the first time, up to the J=82 level. Application of these results to the extension of frequency-comb-based metrology in the infrared is discussed.