In vertebrates, it is known that the hypothalamic-pituitary-adrenal/interrenal (HPA/HPI) is an endocrine axis responsible for steroidogenesis in response to stress. This axis has been highly conserved throughout vertebrate evolution, yet studies on the HPI axis of basal vertebrates are lacking. In this present study, we analyzed the gene expression of presumed HPI signaling genes in a basal bony vertebrate, the Atlantic sturgeon (Acipenser oxyrhincus) and a basal vertebrate, the sea lamprey Petromyzon marinus. We found that both sturgeon and lamprey express a complete array of HPI axis gene components. Importantly, our work is the first to localize expression of steroidogenic acute regulatory protein (star) in interrenal tissue of both of these representative species. Our results provide evidence that even the earliest vertebrates may have a complete HPI axis, and we present how they may be expressing this axis even with a relatively primitive set of genes compared to more modern vertebrates.