As part of a longitudinal study of the New York State Experimental Prekindergarten Program (PreK), several measures were administered to the four-year-old children in the fall of 1975 and to a sample of the same children in the spring of 1976. Analyses of covariance were carried out on three measures of cognitive development with one of five background characteristic variables used as covariate in each analysis. A significant positive effect of the prekindergarten program was found for two of the measures, the Cooperative Preschool Inventory and the Walker Readiness Test. With one exception, the PreK effect was significant over all levels of the control variables; the PreK effect on Walker scores was greater for children whose mothers had less education. The three measures used in the analyses reported here were the Walker Readiness Test, a 50-item, non-verbal measure of general reasoning; the Cooperative Preschool Inventory, a 64-item measure of achievement in several areas regarded as important for success in school; and the Peabody Picture Vocabulary Test, a measure of the ability to understand spoken English. Analyses of covariance of Peabody Picture Vocabulary Test scores did not show a significant PreK effect, even in the presence of the control variables. (Author/MV)