This study of the maturation of prestimulation-induced modulation of startle in 4- to 8-year-old girls and young women demonstrated significant effects of age on both startle amplitude and onset latency modulation. Prestimulation with nonstartling tones resulted in strong inhibition of both amplitude and latency of the startle blink reflex in adults when 25-ms tones preceded the startling stimuli by 120 ms or 250 ms. Following sustained prestimulation for 2000 ms, the adults showed weak nonsignificant response facilitation. Eight-year-old girls showed mature inhibitory startle amplitude modulation, but significantly less inhibition of onset latency compared to adults. Preschool girls showed significantly less amplitude and latency inhibition and more facilitation than 8-year-olds and adult women. These findings in female subjects were very similar to those obtained by Ornitz, Guthrie, Kaplan, Lane, and Norman (1986) in male subjects. Gender differences were limited to the 8-year-old age group. The 8-year-old girls showed significantly less startle amplitude inhibition than 8-year-old boys following the 120-ms and 250-ms prestimulation intervals and less latency facilitation following 2000 ms of sustained prestimulation. In general, these findings suggest that brainstem mechanisms that mediate startle response modulation undergo development during early childhood and do not mature until about 8 years of age in both male and female subjects. The results are discussed in relation to studies of gender effects on development of other neurophysiological variables and to maturation of the nervous system.