One hundred and sixty children 3 1/2-7 years of age (10 M, 10 F at each 6-month interval) were tested on a task that requires inhibitory control of action plus learning and remembering two rules. They were asked to say "day" whenever a black card with the moon and stars appeared and to say "night" when shown a white card with a bright sun. Children < 5 years had great difficulty. They started out performing well, but could not sustain this over the course of the 16-trial session. Response latency decreased from 3 1/2 to 4 1/2 years. Children < 4 1/2 years performed well when they took very long to respond. To test whether the requirement to learn and remember two rules alone was sufficient to cause children difficulty, 80 children 3 1/2-5 years old were tested on a control version of the task ("say 'day' to one abstract design and 'night' to another"). Even the youngest children performed at a high level. We conclude that the requirement to learn and remember two rules is not in itself sufficient to account for the poor performance of the younger children in the experimental condition.