The effect of a positive first-time experience on adrenocortical activity was examined in 48 6- to 13-month-old infants who took part in two sessions of a YMCA-like mother-infant swim class. Experience was manipulated by comparing Novice swimmers with infants who previously had taken a swim class, and by examining responses to first versus second class session. In addition, the effects of temperamental fear of novelty were examined by maternal report prior to the first session. Preswim and Postswim cortisol levels were compared to measures obtained at the same time of day in two comparison groups, one tested at home and one tested in a playroom at the University. The results yielded no evidence of an adrenocortical stress response (elevated cortisol) to the swim sessions for either the Novice or Experienced swimmers. Infant temperamental fearfulness predicted behavioral but not adrenocortical responses during swimming. Postswim cortisol levels were significantly lower than posttest levels in the two baseline comparison groups, and lower postswim cortisol levels were associated with less negative and more positive emotional behavior and engagement of the swimming experience. These results are consistent with the hypothesis that emotions serve as a major pathway influencing adrenocortical activity, and provide no support for the uncertainty hypothesis of adrenocortical activation.