Multicellular organisms typically depend on O₂ for energy production to maintain normal cellular function, and even brief periods of O₂ deprivation may have fatal consequences. The aqueous environment is prone to changes in ambient water O₂ tension (PO₂) and thus the ability of fish to sense changes in water PO₂ and to elicit appropriate physiological responses is essential for their survival. Studies on fish O₂ chemoreception have identified neuroepithelial cells (NECs), which are characterized as having dense-cored vesicles containing serotonin (5-HT), as peripheral O₂ chemoreceptors. Upon exposure to hypoxia, isolated and cultured NECs in vitro depolarize, likely resulting in neurotransmitter release. However, to date there is no evidence that NECs are activated by hypoxia in vivo to initiate physiological responses such as the hypoxic ventilatory response (HVR), which is the focus of this thesis. Initial findings demonstrated that larval zebrafish fine-tune the HVR as early as 4 days post fertilization (dpf) and by 7 dpf, the HVR aids in O₂ uptake under hypoxic conditions. In addition, the HVR is multiphasic, with an initiation phase followed by a decline phase that gradually stabilizes above normoxic baseline values (Chapter 2). In the absence of tools to probe the hypoxia sensitivity of NECs in vivo, research focused on Merkel-like cells (MLCs), a newly proposed O₂ chemoreceptor in larval zebrafish. Using in vivo calcium imaging it was shown that MLCs are stimulated by hypoxia. Data suggest that MLCs are responsible for the initiation phase of the HVR, while peripheral sensory neurons (PSNs)/peripheral sensory ganglia (PSG) that innervate MLCs play a more important role in reducing ventilation during the decline phase of the HVR (Chapter 4). Attempts at identifying the putative neurotransmitter(s) involved in the O₂ signal transduction pathway revealed that adrenaline (AD), serotonin (5-HT), and dopamine (DA) are probable candidates (Chapter 4), though the presence of AD and DA within MLCs is yet to be confirmed. In addition, 5-HT likely plays a role in the central nervous system (CNS), integrating peripheral signals resulting in the final HVR (Chapter 3). Taken together, this thesis provides the first evidence of putative O₂ chemoreceptors responding to hypoxia in vivo and thus significantly advances models for O₂ signal transduction in larval zebrafish.