Ultramicrofluorescence and video fluorescence microscopy are highly sensitive techniques for studying embryo metabolism and developmental potential. These techniques are attractive in that they allow one to study single embryos and thus to detect more subtle changes in embryo metabolism or intracellular pH which, using less sensitive techniques or batches of embryos, would likely go undetected. This is especially important in those species where few embryos can be recovered from a single mother. An additional advantage of ultramicrofluorescence techniques lies in their lack of invasiveness to the embryo under study. When measuring nutrient uptake, several measurements can be made on a single embryo that can then be placed back into culture for evaluation of its developmental potential. Alternatively, test embryos can be transferred back into a recipient for determination of embryo viability. This flexibility further illustrates the beneficial aspects of using ultramicrofluorescence techniques for studying embryo metabolism and development.