The environment sea turtle embryos experience in the nest is determined by the needs of the growing embryos and the ability of the nest substrate to transport heat, water, and respiratory gases to meet those needs. Relatively little is known of the PO₂s in natural sea turtle nests, and what is known has been obtained through methods which may alter the nest PO₂ during measurement. I describe a noninvasive means of measuring sea turtle nest PO₂ using fiber-optic O₂ sensors. I found that both the rate of nest PO₂ decline and the minimum nest PO₂ varied considerably among Loggerhead Turtle (Caretta caretta) nests. Nests exhibited rates of decline and minimum PO₂s that ranged from 0.07 to 0.19 kPa/day and 11.4 to 16.4 kPa, respectively. No difference in minimum nest PO₂ was observed between nests that were relocated and those left in situ. The mean minimum nest PO₂ in this study (13.8 ± 0.21 kPa) is in agreement with other measurements of nest PO₂, but the rate of PO₂ decline measured in this study suggests that previous work may have underestimated how quickly the developing embryos reduce the nest PO₂. The more rapid decline in nest PO₂ may mean that the developing embryos fall below their critical PO₂ and consequently experience reduced metabolic rates much earlier in incubation than previously suggested.