The toxicity of cinnamaldehyde (CNMA) was compared after administration by gavage and in dosed feed. Rats and mice of both sexes received CNMA by daily corn oil gavage (for 2 wk), or in microencapsulated form in feed (2 wk for rats, 3 wk for mice). Feed formulations contained 0-10% CNMA microcapsules, equivalent to approximate daily doses of 0-3000 mg CNMA/kg body weight for rats and 0-10,000 mg CNMA/kg body weight for mice. Concentrations were chosen to deliver CNMA doses approximately equal to doses in the gavage study. Gavage doses of 2620 mg/kg/day and above in mice and 940 mg/kg/day and above in rats produced nearly 100% mortality; there were no deaths in animals receiving microencapsulated CNMA. Rats and mice receiving CNMA in feed showed a dose-related decrease in body weight gain, which was accompanied in rats by hypoplastic changes in reproductive organs and accessory sex glands. CNMA administration by either route caused hyperplasia of the forestomach mucosa. These results demonstrate that microencapsulation in feed can present a useful alternative to gavage dosing for repeated-dose or prolonged-exposure studies, in that (1) the toxic effects of CNMA were similar after gavage dosing and after administration in microencapsulated form in feed, (2) ingestion of chemical in the feed more closely approximates human exposures, and (3) microencapsulation allows the delivery of higher net doses of chemical, while avoiding the acutely toxic effects of a bolus dose.