Auditory feedback (AF), the vocal signals received by the speaker's own auditory system during speech production, plays a vital role in the control of phonation and articulation. It is presumed to help to monitor whether the current utterance is in accord with the speaker's intention. Previous studies have provided evidence that when perturbations are introduced to certain acoustic features of AF, such as fundamental frequency (F0, the perceived pitch), loudness, and formant frequencies, speakers often show compensatory responses, i.e., online articulatory correction in directions opposite to the perturbations. The present study was designed to investigate whether speakers compensate for acoustic perturbations that they are not able to explicitly perceive. Forty native Japanese speakers received two types of modified AF (F0 shift or formant structure manipulation) during sustained vowel articulation. The modifications in F0 (±25, 50 and 100 cents) and formant structure (by contracting or extending the vocal spectrum envelope by 3, 6 and 12 percent) were applied independently. Compensatory responses for F0 were observed in the F0-shifted conditions but not in the formant structure-manipulated conditions even though most of the participants misidentified formant frequency alterations as pitch changes. Furthermore, such F0 compensation showed almost linear relationship with the F0 shifts. Most importantly, compensation was observed even when the F0 shift was very slight (±25 cents) and the participants could not reliably perceive the pitch changes. These results indicate that the self-regulation system of speech production may have an implicit component, which might be founded on an auditory-motor pathway in the cortical language network.