A microwave spectrometer which utilizes the positive column of a dc glow discharge as its sample region has been constructed and operated successfully. This instrument employs free space microwave propagation through a large industrial Pyrex pipe, along with Zeeman modulation, and is intended for the study of transient paramagnetic molecules which are likely to be found in active discharges. This spectromenter features no mechanical tuning, instantaneous broad band width, resolution limited by Doppler broadening, and sensitivity comparable to that of conventional Stark modulated waveguide cells. Extensive observations primarily of the microwave spectrum of the OH radical in a water vapor discharge, have been made in order to delineate the range of application of this technique and to determine the effects of various operating parameters on the spectrum observed. Although the glow discharge is an extremely complicated phenomenon and is capable of manifesting its complexities in the behavior of our spectrometer, a wide range of conditions exist in which the presence of the plasma in the transmission path has negligible impact on the microwave spectrometer signal. Theoretical considerations of basic plasma physics will be employed to support the proposition that this is not unexpected and that a wide range of microwave spectroscopic studies of plasma samples are possible.