A resonator assembly for executing measurements on a sample within a constant magnetic field B.sub.0 by means of magnetic resonance is disclosed. It comprises a resonator portion defining a longitudinal axis and an axial direction. The resonator portion has, along the axial direction, a hollow cavity for exciting electron resonance within the sample. A coupling portion is provided adjacent the resonator portion and has, along the longitudinal axis, a stepped through being electrically conductive at its inner surface. A first, middle section of the stepped through configures the hollow cavity. A second and a third, lateral section adjacent axially opposed sides of the hollow cavity are each dimensioned such that a basic mode being resonant within the hollow cavity is unable to propagate within the second and the third section. A coil is wound around the resonator portion for additionally exciting a nuclear resonance within the sample. The resonator portion comprises a toroidal body made from an electrically conductive material within which there is provided the stepped through. The toroidal body is provided with at least one axial slit.