Prosody plays a variety of roles in infants' communicative development, aiding in attention modulation, speech segmentation, and syntax acquisition. This study investigates the extent to which parents also spontaneously modulate prosodic aspects of infant directed speech in ways that distinguish semantic aspects of language. Fourteen mothers of two-year-old children read a picture book to their children in which they labeled pictures using dimensional adjectives (e.g., big, small, hot, cold). Recordings of the mothers' input to their children were analyzed acoustically and antonyms within each dimension were compared. Mothers modulated aspects of their prosody including amplitude and duration of target words and sentences to distinguish dimensional adjectives. Mothers appear to recruit prosody in the service of word learning. (Contains 1 table, 4 figures and 1 note.)