A method has been devised to measure the uptake of nutrients by small numbers of mouse ova and preimplantation embryos. The 1-5 embryos were incubated as a group at 37 degrees C in a small volume (at least 20 nl/embryo) of culture medium on siliconized microscope slides under mineral oil. Serial 4 nl samples were taken at 30-min intervals for 3 h and assayed for their content of glucose and pyruvate using an ultramicrofluorometric technique. Pyruvate uptake exceeded that of glucose in unfertilized and fertilized ova and in the developmental stages up to the blastocyst, when glucose became the predominant substrate. A component of the uptake of pyruvate by unfertilized ova was probably mediated by a carrier.