Three isoelectronic pairs of compounds, N-formyl- and N-nitrosopyrrolidine, piperidine, and morpholine, have been characterized by microwave spectroscopy. Only one conformational isomer is observed in each compound. Both the inertial constants and the 1 4 N quadrupole coupling constants determined here show that the molecular geometries of each isolectronic pair are extremely similar. Rotational spectra of vibrationally excited species of N-nitrosopyrrolidine and N-nitrosopiperidine are assigned. The range of structural variation among the vibrationally excited states is comparable to the structural variation within the isoelectronic pairs of compounds, suggesting that a biological receptor for one compound is likely to bind its isoelectronic analog as well. Ab initio molecular orbital calculations of N-formylpyrrolidine at the MP2/6-31G * * //MP2/-31G * * level reproduce the observed rotational econstants extremely well with a model that includes essentially planar bonding about the amino N atom.