Using DNA excess filter hybridization to pulse-labeled cellular RNA, we examined the stability of vitellogenin mRNA in Xenopus liver in relation to estrogen concentration. We showed that pharmacological concentrations of estrogen stabilize vitellogenin mRNA against degradation but that physiological concentrations do not. We concluded that there is little foundation for the common belief that estrogen stabilizes vitellogenin mRNA in normal liver cells and that such stabilization contributes to the normal expression of vitellogenin genes. We also discuss the importance of steroid concentration in other contexts, and show that the widespread tendency to use artificially high concentrations may lead to questionable conclusions.