Results of a series of studies suggested that excessive early vocal stimulation may be detrimental to infants' cognitive processing, and that optimal development may be associated with a moderate amount of early stimulation. Study 1 involved 59 3-month-old infants and their mothers. Study 2 observed 23 mothers and infants seen 3 months after the child's due date. Study 3 extended the sample to include 31 3-month-old Greek infants and their mothers from intact Greek homes. And study 4 further extended the sample to include 34 3-month-old infants and their mothers who came from diverse socioeconomic and ethnic backgrounds. All four studies found a curvilinear relationship between infants' differential vocal responsiveness (DVR) and the quantity of maternal vocal stimulation (MVS). In each study mothers were rank-ordered according to how much time they had spent talking to their infants during naturalistic sessions and then were separated into high, middle, and low MVS groups. The mean value of infants' DVR was calculated for each group. Differences between the three groups' DVR scores were statistically significant in each study. Low DVR was associated with both too little and too much MVS, while maximum DVR was associated with a moderate amount of MVS. Findings of an analysis of variance on the combined sample for MVS groups in mother- and stranger-present conditions are reported and discussed. (RH)