ABSTRACT This study intended to investigate the determination of activities and involvement of mentally challenged children in regular classes in Ilmashariani Zone, Central Division, Narok district. The most important and controversial issues in this study is the extent to which children with special needs should and can be involved in regular classrooms. It should be noted that special needs education has moved historically from total absence, separate provision, partial integration to the inclusion stage. (Clark 1997). Less attention was placed on transferring children with special needs from special to mainstream schools. However this is not the case today both a third world and developed countries. Kenya in particular, the introduction of UPE saw the increased enrolment of mentally retarded children into the education system which had a direct impact on the teachers' way of handling, assessing and evaluating the activities of their learners. Hence the research was carried out putting much emphasis on teacher's ability to identify the children with mental problems, teachers' attitudes towards inclusion of children who are mentally challenged, and to assess how teachers determine and involve mentally challenged children in the teaching-learning process in regular schools.