As part of an investigation of the developmental effects and structure-activity relationships of aliphatic acids, 5-methylhexanoic acid was administered by gavage to Sprague-Dawley rats on gestation days 6-15 at doses of 0, 300, and 400 mg/kg/day. The dams were allowed to deliver and their litters were examined through postnatal day 6. Pups that were found dead were examined for soft-tissue alterations. On day 6, two survivors per litter were preserved for skeletal examination. Maternal toxicity was demonstrated at both 300 and 400 mg/kg by reduced weight gains, altered respiration (rales, dyspnea) and death. Body weight losses early in the treatment period were also evident at 400 mg/kg. In spite of the marked maternal toxicity present, there were no clear developmental effects; litter size, pre- and postnatal viability, and pup weights were unaffected by treatment. Skeletal examinations of selected pups also revealed no compound effects.