Effects of elevated maternal body temperature upon incorporation of an RNA precursor by rabbit eggs were evaluated by culture in vitro at 39·0 or 40·3°C. temperatures comparable to rectal temperatures of rabbits in normal and elevated ambient temperatures. One-cell fertilized eggs incorporated significantly less [3H]uridine during culture at the higher temperature, indicating a depression of RNA synthesis. Uridine incorporation was not significantly decreased when two-cell ova were incubated with [3H]uridine after completion of the first cleavage, but was decreased in two-cell ova that had been subjected to the higher temperature only while in the one-cell stage of development. The observed reductions in uridine incorporation at the higher temperature were not accompanied by a decrease in energy production as determined by CO2 production from [14C]pyruvic acid. These results suggest that such alterations of regulatory function are associated with the sensitivity of the early zygote to thermal stress and is a mechanism by which elevated maternal body temperatures adversely influence subsequent embryonic development.