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The rotational spectra of the nitrogen bases uracil}, \textbf{110}, 2329 (1988)} (m.p. 335 C) and thymine}, 37, (1989)} (m.p. 316 C) have been observed in the 6-18 GHz frequency range by using a Laser-Ablation Molecular Beam Fourier Transform Microwave Spectrometer (LA-MB-FTMW).}, \textbf{74}, 4799 (2003).} The nuclear quadrupole coupling hyperfine structure corresponding to the presence of two $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$N atoms has been completely resolved for both molecular systems. The rotational spectrum of all uracil $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$C and $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}$N monosubstituted isotopomers in their natural abundance are being observed. Following the characterization of these isotopomers, the uracil structure in gas phase will be obtained.