Hepatocytes isolated from livers of Fischer 344 rats and transplanted into the spleens of rats from the same strain survived for at least 15 mo in the absence of immunosuppressive drugs. Hepatocytes attached themselves only in the red pulp of the spleen, most commonly in clumps without a discernible structure. Throughout the 15-mo period, intrasplenically transplanted hepatocytes expressed cytochrome P-450b, P-450e and albumin messenger RNAs, whereas alphafetoprotein messenger RNA was not expressed. In addition, the relative expression of albumin and P-450 genes was similar to that in liver. For example, albumin messenger RNA was expressed to higher levels than P-450b or e messenger RNAs. Northern blots hybridized with oligonucleotides specific for P-450b or P-450e showed that, as in liver, both P-450b and P-450e genes were induced in response to phenobarbital. Quantitative slot-blot hybridizations performed at 15 days and 1, 6, and 15 mo after hepatocyte transplantation revealed that cytochrome P-450b and P-450e messenger RNAs were induced about 20- to 30-fold by a single dose of phenobarbital. This level of induction was also similar to that observed in liver. Hence, intrasplenically transplanted hepatocytes represent a unique system in which hepatocytes, cultured in an extrahepatic in vivo environment, maintain for at least 15 mo a pattern of expression for these four liver genes similar to that in the adult liver. Moreover, these studies suggest that neither the organization of liver into acini nor a specific zonal sinusoidal microenvironment is necessary for adult hepatocytes to respond to phenobarbital with induction of P-450b and P-450e genes.(HEPATOLOGY 1990; 11:585:593.)