Assessing the impact of disease and treatment of the emotional state and temperament of preschool children has been limited by the lack of sensitive and objective measurement techniques. To construct such a measure for longitudinal use, a sample of 179 children with febrile seizures and 85 normal children were used to develop the Minnesota Preschool Affect Rating Scales (MN-PARS). Video-taped play sessions were a source of behaviorally anchored ratings on 12 scales. Factor analysis yielded three factors of Negative Affect, Positive Affect, and Self-regulation with additional individual scales of Dependency and Activity Level. These scales and factors yield reliable ratings, as measured by interrater agreement and split-half techniques, as well as initial evidence of concurrent validity. Although they were developed for measuring the behavioral effects of phenobarbital on children with febrile seizures, these scales provide an objective means of measuring emotional expression and self-regulation useful for other studies.