Formic hydrazide (formylhydrazine) has been investigated by microwave spectroscopy in the 8-62 GHz spectral range, as well as by quantum chemical ab initio and density functional theory calculations made at several levels of theory. Both the ab initio and the density functional theory calculations predict that two stable forms exist for this compound, a more stable conformer having a syn-periplanar arrangement for the heavy atoms, and a less stable form where these atoms are anti-periplanar. The latter rotamer is calculated to be 10-14 kJ/mol less stable than the former, depending on the theoretical approach. The barrier height separating the two forms is calculated to be 92.2 kJ/mol at the B3LYP/cc-pVTZ level. The microwave spectra of the ground and four vibrationally excited states of the lowest torsional mode of the syn-periplanar conformer were assigned. The lowest torsional frequency was determined to be 77(15) cm(-1) by relative intensity measurements. The variation of the rotational constants upon excitation of this mode and the lowest torsional frequency were fitted to a potential function of the form V(z) = 15.6([z(4)] + 4.9[z(2)]) cm(-1), indicating that the heavy atoms are effectively planar at the equilibrium conformation. The dipole moment was determined to be mu(a) = 1.54(2), mu(b) = 1.85(2), mu(c) = 0 (assumed), and u(tot) = 2.41(3) D [8.04(8) x 10(-30) C m] by Stark effect measurements.