Three studies were conducted with three groups of eighteen autistic children (3-11 years old) matched by age and IQ to developmental aphasic Ss to examine the nature of the cognitive deficit underlying autism. Two experiments were concerned with language comprehension using real and anomalous sentences, while the third investigation was an observational study of the play of autistic children. In experiment I, autistic Ss were found not to use a probable event strategy in sentence comprehension. In experiment II, autistic Ss demonstrated ability to extract and apply a linguistic rule (word order) but failed to apply a non-linguistic rule (probable event). In experiment III, the majority of the Ss spent most of their time engaging in simple undefined manual activity or repetitive manipulation with toys, with relatively few examples of relational or functional play. Data from all three studies were consistent with the hypothesis that autistic children have a specific cognitive deficit in understanding relational aspects of the environment, particularly for the probabilities involved in such relationships among people and objects. (CL)