Body mass, length, oxygen consumption (M˙o ₂) and heart rate (f _H) were measured in “embryos” (prior to hatching), “larvae” (days 10–20), “juveniles” (days 30–70 in 10-day intervals), and “adults” (day 100) of the zebrafishDanio rerio. Fish were chronically reared at either 25, 28, or 31°C and then acutely exposed to hypoxia at different developmental stages. We hypothesized that at any given rearing and measurement temperature,D.rerio would maintainM˙o ₂at lower ambient P O 2 [i.e., have a lower critical partial pressure (P_crit)] as development progressed and that at any given developmental stage individuals reared and measured at higher temperatures would show a more pronounced hypoxic bradycardia.M˙o ₂in normoxic fish at 28°C peaked at ∼40 μmol ⋅ g⁻¹ ⋅ h⁻¹at day 10, thereafter falling to 4–5 μmol ⋅ g⁻¹ ⋅ h⁻¹at day 100. The Q₁₀ forM˙o ₂was 4–5 in embryos, falling to 2–3 fromday 10 today 60 and rising again to 4–5 atday 100. P_crit at 28°C was ∼80 mmHg in embryos but decreased sharply to 20 mmHg at 100 days, supporting the hypothesis that more mature fish would be better able to oxygen regulate to lower ambient P O 2 levels. P_crit increased sharply with measurement temperature. Heart rate (f _H) at 28°C increased from about 125 beats/min in embryos to a peak of ∼175 beats/min at days 10–30and then fell to ∼130 beats/min byday 100. Unlike forM˙o ₂, the Q₁₀ forf _H was more constant at 1.2–2.5 throughout development. Hypoxic exposure at any temperature had no effect onf _H until ∼day 30, after which time a hypoxic bradycardia was evident. As evident forM˙o ₂, the bradycardia in older larvae was more profound at higher temperatures. On the assumption that bradycardia is indicative of hypoxic stress, the increasing prevalence of a hypoxic bradycardia in older, warmer individuals supports the hypothesis that increasing hypoxic susceptibility with development would be exacerbated by increasing temperature. Collectively, these data indicate that the ability to regulateM˙o ₂and f _H in response to the compounding demands of increased temperature and/or decreased oxygen availability first develops after ∼20 days in D.rerio and, thereafter, the ability to maintainM˙o ₂in the face of ambient hypoxia progressively builds through to adulthood. Additionally, the temperature responses of metabolism and heart rate differ substantially at different phases of development, suggesting a loose coupling between the respiratory and cardiovascular systems, at least early in development.