The rotational spectrum of jet-cooled tetrahydrofuran has been investigated in the 8-18 and 60-78 GHz frequency ranges. Only the four lowest vibrational levels remain populated in the jet conditions. Two pseudorotation vibrational spacings have been measured directly to be Delta E-01=21 307.71(3) MHz and Delta E-23=61 205.69(3) MHz. They have been used, together with the lowest 10 measured far infrared transition frequencies and with the effective moments of inertia of the ground state and their shifts upon excitation observed in this work, to determine the potential energy function for pseudorotation and the associated structural relaxations. The potential energy barriers are 45 cm(-1) at the envelope conformation of the oxygen and 16 cm(-1) at the twisted conformation of the C-beta-C-beta' bond relative to the CalphaOCalpha' plane. The four symmetrically equivalent equilibrium structures are close to the envelope conformations of the CH2 groups next to the oxygen atom. While the C-O bond lengths and the local CH2 angles appeared to remain unaffected by pseudorotation, significant changes found for the diagonal O ... C distances as well as for the C-C and C-H bond lengths are of interest as possible effects of hyperconjugation.