Rat embryos explanted at head fold stage were stored under various levels of hypothermia prior to culture. The storage media were Hanks' Balanced Salt Solution (BSS), 50% rat serum with 50% Dulbecco's Modification of Eagle's Medium (standard medium), or 100% rat serum. The media were gassed with 5% O2/5% CO2/90% N2 or 20% O2/5% CO2/75% N2. Subsequent development of embryos after storage at temperatures between 10 degrees C and 30 degrees C for 5 hr in Hanks' BSS, or for 5-10 hr in standard medium or serum, was similar to that of controls. Some embryos developed well even after storage for 48 hr in standard medium. Development was poorer after storage at 0 degrees C or 5 degrees C, and after storage at all temperatures in ungassed Hanks' or standard medium (pH greater than 8.0). Differences in oxygen level had little effect. For routine explantation at room temperature in (ungassed) phosphate-buffered saline solutions such as Hanks', it is recommended that the delay before transferring the embryos to the culture incubator not exceed 2-3 hr.