Industrial, agricultural, and pharmaceutical requirements drive the development of a plethora of new chemical entities each year, many of which - for example drugs, pesticides, and food additives - have to be assessed for potential human health hazard. The current benchmark for risk assessement is the lifetime rodent bioassay which is expensive, time-consuming, laborious, requires the sacrifice of numerous animals, and is often irrelevant to humans. Hence alternative strategies to the rodent lifetime bioassay for prediction of chemical carcinogens are being pursued, especially for the liver which is an organ frequently affected by exogenous chemicals due to its detoxifying and metabolic roles. Numerous studies in recent years support the important role of microRNAs in cancer development, including hepatocellular carcinoma. The principal hypothesis of this project was that hepatic microRNA signatures can contribute to the earlier prediction of chemical hepatocarcinogens. Examination of livers from male Fischer rats treated with six chemical hepatocarcinogens, with diverse mode of actions for 90 days revealed that all the tested hepatocarcinogens affected the liver miRNAome from that early stage. Interestingly, a small set of microRNAs were identified whose expression was frequently deregulated by the hepatocarcinogens. Bioinformatic analysis indicates that these microRNAs can regulate pathways which are important in hepatocellular carcinoma. A more detailed investigation of one of those hepatocarcinogens, phenobarbital, showed that its effects on liver microRNAs were both dose and time dependent, with a progressive induction of specific microRNA clusters.Thus this study was the first to investigate in depth the effects of chemical hepatocarcinogens on the liver miRNAome and supports the potential usefulness of hepatic microRNA signatures in risk assessment.