A relaxation oscillator which is suited for fabrication as a monolithic circuit and which uses a parallel resistive capacitive frequency determining network wherein a capacitor is charged and discharged between an upper and lower voltage level. As the capacitor is charged to a potential exceeding a first threshold voltage level supplied to a comparator switch, the operating state of the comparator is caused to switch. A current detecting circuit is included which detects a current that is proportional to the charging current supplied to the capacitor from a charge circuit. As the proportional current decreases in value below a predetermined level due to the capacitor being charged to the upper voltage level the current detecting circuit is disabled which actuates a control circuit for switching the threshold voltage applied to the comparator to a lower level. The actuated control circuit also disables the charge circuit such that the capacitor discharges to the lower voltage level at which time the comparator switches states to disable the control circuit whereby the threshold voltage level is switched to the first level. Thereafter, the charge circuit is enabled to enable the current detecting circuit and to charge the capacitor to the upper voltage level.