Previous studies have demonstrated that ingestion of 5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione (oltipraz) during the aflatoxin B1 (AFB1) treatment phase completely prevented hepatic cancer. In this study we evaluated the effect of feeding oltipraz during the post-AFB1 treatment phase. Fifty-five male F344 rats were divided into five groups. All rats were gavaged with 25 micrograms AFB1/rat, five times a week for two successive weeks. The rats were fed the oltipraz-supplemented diet according to three different feeding regimes: during the AFB1 treatment phase (1 week prior to, during and 1 week after the last gavage with AFB1); during the post-treatment phase; or throughout the entire time of the experiment. Phenobarbital-supplemented diet was fed during post-treatment phase to one group and this was used as a positive control for the promotion of AFB1-induced focal growth. The burden of putative, preneoplastic, hepatic glutathione S-transferase P-positive foci was evaluated at 13 weeks after the AFB1 treatment phase. As seen previously, oltipraz fed during the AFB1 treatment phase significantly inhibited focal development, i.e. the volume percent of the liver occupied with foci was reduced by 87%. Oltipraz when fed during the post-treatment phase neither inhibited nor enhanced focal development.