Three- and four-month-old infants were tested on their ability to discriminate the third-formant transitions sufficient to signal the syllable-medial stops [t] and [k]. The stimulus patterns consisted of an initial fricative [s], 20 or 100 msec of silence, and the vowel [a], whose initial formant transitions were appropriate for [t] or [k]. Discrimination only occurred when the duration of silence was 100 msec. This constraint on discrimination is discussed in terms of a psychoacoustic explanation based on forward masking and in terms of the hypothesis that the processing of speech signals involves a species-specific system dedicated to deriving a phonetic message.