Aided by a consensus of member companies, methyl chloride was selected by CIIT as a commodity chemical which required comprehensive investigation of possible toxic and carcinogenic effects during chronic inhalation exposure in mammals. Such an investigation was planned and began with a ninety-day pilot study. The design and results of this latter study are the subject of this report. The Fischer-344 inbred rat and the BC Fl hybrid mouse were selected as the species and strains of choice because of a long life span; moreover, the widespread acceptance and usage of these animals in long-term toxicology research has generated a considerable background of data on spontaneous changes. This data could be employed in the future to resolve questionable toxicities. Inhalation exposure for 6 hours a day, 5 days each week, was used to stimulate the conditions of occupational usage of this agent. Decisions regarding the exposure levels to be administered during the initial pilot study were based upon information obtained from the literature or outside sources on file at the CIIT library.