When listeners process segmentally relevant properties of the speech signal they do so in a rate-dependent manner. This is seen as a shift in the perceptual category boundary; as rate declines and overall syllable duration increases, the category boundary moves toward a longer value of the acoustic property in question. Focusing on the /b/-/p/ distinction specified by voice onset time (VOT), we investigated whether the acoustic modifications that occur with an alteration in speaking rate accord with this pattern of perceptual boundary shift. Two main findings emerged. First, as speaking rate became slower and overall syllable duration became longer, the VOT value of the consonant, especially that of the voiceless /p/, also became longer. Second, and most important, the VOT value that optimally separated the /b/ and /p/ VOT distributions also changed with rate, increasing with increasing syllable duration. However, the magnitude of the boundary shift obtained for these production data was greater than that typically found in perceptual experiments. This suggests the existence of constraints on the extent to which the perceptual system can accommodate for alterations due to rate of speech.