Irritability levels and activity reactivity to aversive tactile stimuli were compared for 144 full-term neonates and 191 preterm infants. Irritability ratings increased across the five trials both during and post stimulation for full-term females and males and for preterm females, but not for preterm males. Activity ratings decreased across trials during stimulation for all groups except preterm males, and post stimulation for all groups. Females were more irritable than males during stimulation and full-term infants were more active than preterm infants on the initial trial. No other sex or term group differences were observed during or post stimulation. Findings have implications for future studies of infant brain development and stimulation. (Author/AA)