Two processes strongly influence the distribution of oxygen within egg masses and cells: the supply of oxygen by diffusion and the consumption of oxygen by embryos and mitochondria. These processes are differentially sensitive to temperature. The diffusion coefficient of oxygen depends only weakly on temperature, having a io of approximately 1.4. In contrast, the consumption of oxygen depends strongly on temperature, having a io between 1.5 and 4.0. Thus, at higher temperatures, the ratio of oxygen supply to demand decreases. I show, by extending a model of oxygen distribution within metabolizing spheres, that maximal egg-mass sizes and cell sizes are predicted to be smaller at higher temperatures. For egg masses, definitive data are not yet available. For ectotherrnic cells, this prediction appears to be supported; cells from a variety of ectothermic organisms, unicellular and multicellular, are smaller when the cells are produced at warmer temperatures. Establishing a specific connection between this pattern and oxygen distributions requires demonstration of (1) oxygen concentration gradients within metabolizing spheres and (2) central oxygen concentrations low enough to affect function. Egg masses from a variety of taxa show steep oxygen concentration gradients and often are severely hypoxic or anoxic in central locations. Severe hypoxia appears capable of retarding development or killing embryos. Similar kinds of data for ectothermic cells have not yet been collected, but the literature on oxygen gradients within mammalian cells suggests that intracellular gradients may be important.