The theory of an optical parametric oscillator located internal to the cavity of the pump laser has recently been presented by Oshman and Harris. An oscillator of this type has been constructed and the details of its operation are presented here. Several features predicted by the theory have been observed; in particular a form of relaxation oscillation peculiar to the internal oscillator has been observed for the first time. In addition to the experimental results the question of the stability of the various modes of operation and factors affecting the pulse repetition rate of the pulsing solution are considered.