The rotational spectra of five isotopologues (normal and all monosubstituted C-13 species) of methylpyruvate have been measured with the pulsed jet Fourier transform microwave technique. Rotational transitions are split into quintets due to the internal rotations of the two methyl groups. The corresponding barriers to internal rotation have been determined to be V-3(H3C-O) = 4.883(8) kJ mol(-1) and V-3(H3C-C) = 4.657(8) kJ mol(-1), respectively. Information on the skeletal heavy atom structure has been obtained from the 15 available rotational constants.