Steroid hormones have some unusual effects in hamsters. We have been studying the endocrine control of a protein synthesized in liver of the Syrian hamster. This protein is homologous to human C-Reactive protein (CRP) and amyloid P component (AP) sharing a 50-70% (respectively) identical amino acid sequence and having a similar molecular configuration; that is, an oligomer of 5 identical monomer proteins noncovalently assembled as a cyclic pentamer. Furthermore, FP shares similar functional properties with CRP + SP such as Ca++ dependant Phosphoryl-choline and galactan binding, complement fixation, and acute phase responsiveness. We have previously shown that FP is a constituent of hamster amyloid, and high serum levels of FP appear to have a primary role in the deposition of amyloid in the Syrian hamster. So far, only in hamster is pentraxin synthesis controlled by sex hormones so that females have 200-300 fold more FP in serum than males. This unusual endocrine control suggests that FP in the hamster may have some unusual function which could provide information about the general function for all pentraxins. We have examined other members of the hamster family to determine if endocrine control of pentraxin synthesis is a general feature in these rodents. FP synthesis in Armenian hamster is under a very different control mechanism, because estrogen administration actually decreases FP synthesis in direct contrast to effect of estrogens in Syrian (male) hamsters. Of particular interest is the acute hepatotoxicity induced in Armenian hamsters by exogenous estrogen which is detectable histologically by hepatocellular degeneration. Chronic administration of diethylstilbestrol (DES) is associated with the appearance of hepatocellular carcinomas. Recently we have observed the formation of neoplastic lesion in liver after chronic treatment with Zeranol, and estrogen produced by fungi and frequently found as a contaminant of food. Although Zeranol is known as a relatively weak estrogen, the effect of Zeranol on liver of Armenian hamster is much greater than expected. The indication of hepatic tumors by estrogen alone is unusual, and the Zeranol-hamster model is a particularly suitable experimental animal for studies of hormone-initiated carcinogenesis.