Discrimination of two acoustic dimensions of auditory stimuli, vowel identity and pitch contour, was tested with infants between the ages of 4 and 16 weeks using the high-amplitude sucking (HAS) technique. Discrimination of the vowel dimension and the pitch dimension was tested under two conditions: when a change in the target dimension occurred in the absence of constant variation in a second dimension, and when a change in the target dimension occurred in the presence of constant variation in a second dimension. In addition, discrimination was tested in a combined condition in which one level of the vowel dimension was always combined with one level of the pitch dimension and the stimulus change to be detected was a recombination of the levels of each dimension. Sucking-recovery scores demonstrated that infants always discriminated a change in the target dimension when it occurred without variation in the second dimension, regardless of the dimension that served as the target. However, while variation of the pitch dimension did not alter vowel discrimination, variation in the vowel dimension interfered with discrimination of the pitch dimension. Discrimination was also not evidenced in the combined condition. Analysis of the group time-to-habituation (TH) data revealed that significantly longer TH scores were correlated with a failure to demonstrate discrimination. The data are discussed in terms of the formation of auditory perceptual categories in early infancy as they relate to the acquisition of speech and language and, more generally, to developmental attention and memory for auditory stimuli.