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Adaptive thermal tolerance plasticity can dampen the negative effects of warming. However, our knowledge of tolerance plasticity is lacking for embryonic stages that are relatively immobile and may benefit the most from an adaptive plastic response. We tested for heat hardening capacity (a rapid increase in thermal tolerance that manifests in minutes to hours) in embryos of the lizard Anolis sagrei. We compared the survival of a lethal temperature exposure between embryos that either did (hardened) or did not (not hardened) receive a high but non-lethal temperature pre-treatment. We also measured heart rates (HRs) at common garden temperatures before and after heat exposures to assess metabolic consequences. 'Hardened' embryos had significantly greater survival after lethal heat exposure relative to 'not hardened' embryos. That said, heat pre-treatment led to a subsequent increase in embryo HR that did not occur in embryos that did not receive pre-treatment, indicative of an energetic cost of mounting the heat hardening response. Our results are not only consistent with adaptive thermal tolerance plasticity in these embryos (greater heat survival after heat exposure), but also highlight associated costs. Thermal tolerance plasticity may be an important mechanism by which embryos respond to warming that warrants greater consideration.