Functional and referential changes in maternal speech were investigated in a developmental study of nineteen mother–infant dyads, using video recordings of their free play at three and six months. The role of the infant in influencing speech adjustments was investigated by analysing the relationships between different types of maternal speech and different infant behavioural modes. Three general modes were differentiated – Communicative, Praxic and Other – and, regardless of infant age, mothers were found to make some modifications to their speech style as a function of infant mode. Speech style also was found to change, at a pragmatic level, from three to six months. Both findings support the conclusion that maternal speech is influenced by non-linguistic behavioural ‘feedback’ from infants. However, a finding that affect-oriented speech is more sensitive to infant behaviour than informative speech supports Brown's (1977) contention that the maternal speech register is shaped by two relatively independent interpersonal functions – the affective and communicative components. It is argued that a fuller account of maternal conversational adjustments to prelingual infants requires both a dialogic and a monologic explanation.