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Identification and discrimination of two-formant [bae-dae-gae] and [pae-tae-kae] synthetic speech stimuli and discrimination of corresponding isolated second formant transitions (chirps) were performed by six subjects. Stimuli were presented at several intensity levels such that the intensity of the F2 transition was equated between speech and nonspeech stimuli, or the overall intensity of the stimulus was equated. At higher intensity (92 dB), b-d-g and p-t-k identification and between-category discrimination performance declined and bilabial-alveolar phonetic boundaries shifted in location on the continuum towards the F2 steady-state frequency. Between-category discrimination improved from performance at 92 dB when 92-dB speech stimuli were simultaneously masked by 60-dB speech noise; alveolar-velar boundaries shifted to a higher frequency location in the 92-dB-plus-noise condition. Chirps were discriminated categorically when presented at 58 dB, but discrimination peaks declined at higher intensities. Perceptual performance for chirps and p-t-k stimuli was very similar, and slightly inferior to performance for b-d-g stimuli, where simultaneous masking by Fl resulted in a lower effective intensity of F2. The results were related to a suggested model involving pitch comparison and transitional quality perceptual strategies.In categorical perception, a listener's ability to discriminate between two speech sounds is determined by his or her classification of the sounds into phonetic categories. When the two sounds are classified into different phonetic categories, they are highly discriminable, but when the two sounds are classified into the same phonetic category, even though the physical difference between the stimuli is the same as that of the between-category stimuli, they are discriminated at a very low level.