This study examined the expression of EPO, VEGF and VEGF receptor gene under conditions of reduced oxygen supply in primary cultures of rat hepatocytes, and compared it with the expression of these genes in hypoxic rat livers in vivo. To this end we exposed male Sprague-Dawley rats to hypoxia (10% and 8% O2), carbon monoxide (0.1% CO) or injected cobalt chloride (60 mg/kg CoCl2) subcutaneously. For the in vitro experiments we used primary cultures of rat hepatocytes which were kept at high (20% O2) and low (1% O2) oxygen tensions for three hours. The EPO mRNA was up-regulated by hypoxia in vitro and in vivo about 10-fold. The VEGF mRNA was up-regulated fivefold in the hepatocytes only, whereas the in vivo mRNA levels remained unchanged. The mRNA levels of flt-1 were up-regulated threefold by 8% O2 in livers, dependent on the strength of hypoxia (10% caused no changes in flt-1 gene expression) and on the kind of hypoxic stimulus (8% O2 was as effective as 0.1% CO and more effective than cobalt). The mRNA levels of flk-1/KDR and flt-4 remained unchanged in the liver. In vitro there were no changes in the mRNA levels of flt-1, flt-4 and flk-1/KDR. Consequently, the in vivo regulation of VEGF, which might be modulated by induction of flt-1 receptor gene expression, differs from the in vitro cell culture situation and might be different from the EPO regulation in vivo.