A method of determining the efficacy of secretin, neuropeptides, peptides, and/or digestive enzymes for treatment of an individual diagnosed with a pervasive development disorder, such as autism, where the individual has an abnormal fecal chymotrypsin level and exhibits a lack of protein digestion. A sample of feces is obtained from the individual in order to determine a quantitative level of chymotrypsin in the sample. The quantitative level of chymotrypsin is used to determine the efficacy of treating the individual with secretin, neuropeptides, peptides, and/or digestive enzymes. If it is determined, based on the individual's fecal chymotrypsin level, that the individual would benefit from the administration of secretin, neuropeptides, peptides, and/or digestive enzymes, then the amount necessary to effect a change in the individual's autistic behavior is determined by the individual's fecal chymotrypsin level, age and weight.
