Embryonic hemoglobin circulated by the developing heart in the early vertebrate embryo is widely assumed (without substantiation) to perform the same vital role of O₂ carriage that it does in fetuses and adults. In order to challenge this assumption, we measured highly O₂-dependent physiological variables like O₂ consumption, cardiac performance, and initial swim bladder filling in the presence and absence of functional hemoglobin in the embryos and early larvae of the zebra fish, Danio (=Brachydanio) rerio. Functional ablation of hemoglobin by carbon monoxide or phenylhydrazine did not reduce whole-animal O₂ consumption, which was ≈85 to 90 μmol·g⁻¹·h⁻¹. Similarly, no differences in heart variables like ventricular pressure development or heart rate, which increased from 135 to 175 bpm between stages 36h and 96h (indicating developmental stages 36 and 96 hours after fertilization, respectively), were observed in these experiments. Initial opening of the swim bladder was not influenced in the presence of CO-occupied hemoglobin but was significantly impaired when the embryonic hemoglobin was chemically modified by incubation with phenylhydrazine. That aerobic processes continue without hemoglobin O₂ transport indicates the adequacy in the embryo of simple O₂ diffusion alone even in developmental stages with extensive convective blood circulation generated by the heart.