Monoamine oxidase specific activities against PEA and 5-HT have been measured in mitochondria isolated from early embryos of Bufo bufo. During the early development up to the neural fold stage, MAO activity undergoes a continuous decrease that is more evident when PEA is used as the substrate. The inhibition patterns of deprenyl and clorgyline demonstrate that, at the neural fold stage, both type A and B MAO are present. Both in eggs and embryos MAO type A activity appears slightly more sensitive to the inhibitory effect of various concentrations (0.1-2 M) of the denaturing agent urea.