The research used three scales originally developed by Grisso to measure competency to waive such rights in juveniles. One sample of 18 males and 11 females worked in a sheltered workshop setting; the other consisted of 21 male and four female probationers. Results revealed that both samples scored substantially lower on measures of Miranda comprehension than Grisso's samples of juveniles and adults. Differences also existed between the samples that appeared attributable to differential levels of criminal justice experience. Findings indicated that the Grisso scales may be helpful in determining competency to waive Miranda rights in defendants with mental retardation. In addition, the data raise strong concerns about the ability of many defendants with mental retardation to make intelligent Miranda waivers. Findings indicate the need for warnings in a simplified manner with more explanation. In addition, criminal justice personnel should check defendants for comprehension using questioning strategies similar to those used in the Grisso tests. Tables and 25 references (Author abstract modified)