Ethyl acrylate (EA) is a known forestomach carcinogen in both rats and mice. Recent work in this laboratory indicated that carcinogenic doses of EA administered by gavage for 13 wk resulted in a sustained increase in forestomach epithelial hyperplasia as long as exposure to EA continued. However, hyperplasia regressed and no forestomach neoplasms were seen after a 19-mo recovery period. Current studies were designed to further investigate the time required for sustained hyperplasia to lead to neoplasia as well as the organ specificity of EA-induced cell proliferation/hyperplasia vs carcinogenicity. EA was administered at 200 mg/kg (po) to male Fischer-344 rats, 5 days/wk. Squamous cell proliferation/hyperplasia was observed in the forestomach of all rats that received EA for 6 or 12 mo. Treatment of rats with EA for 12 mo followed by 2 mo of recovery resulted in the development of forestomach papillomas in 2 of 5 treated rats. Furthermore, animals treated for 12 mo and allowed 9 mo of recovery exhibited an increase in forestomach squamous cell carcinomas and papillomas at a combined incidence of 4 in 13. In contrast, animals treated with EA for 6 mo and allowed 2 or 15 mo of recovery exhibited a time-dependent regression of cell proliferation and did not develop forestomach neoplasms. No significant elevation in liver cell proliferation or neoplasia was seen at any time in any of the rats included in the present study, further confirming the organ specificity in the relationship between EA-induced cell proliferation and carcinogenicity. In conclusion, EA resulted in increased cell proliferation in the target organ of carcinogenicity (forestomach) but not in nontarget organs such as the liver. This work indicates that cell proliferation, sustained for a sufficient period of time, results in the development of neoplasia despite cessation of chemical administration. Furthermore, a temporal relationship exists between EA-induced epithelial cell proliferation and forestomach carcinogenicity.