Summary The free amino acids and the amino acids of peptides and soluble and insoluble proteins have been quantitatively determined by microbiological assay of extracts of 22 stages of early development and 4 ‘stages’ of aged unfertilized eggs of the sea urchin Paracentrotus lividus. In any given fraction, the 17 amino acids assayed show a remarkably uniform developmental pattern, although large concentration changes take place and the patterns of changes are different for the different fractions. The changes are generally abrupt, of short duration, synchronous, and either complementary or supplementary to one another. These changes appear to be caused by the degradation of reserve yolk proteins to free amino acids and peptides and the synthesis of new embryonic proteins therefrom. The developmental changes in the peptide (or bound) amino acids are very similar to those of the free amino acids. There are at least 3 major and 3 minor periods of new protein synthesisand 4 periods of intense yolk-protein breakdown. Yolk-protein degradation and the synthesis of new embryonic proteins take place at more or less different times, so that the changes in the protein and non-protein amino acids occur in complementary cyclic waves. Protein synthesis takes place in unfertilized eggs which are allowed tostand in sea water, and the non-protein amino acids are depleted to a relatively low level. This is probably a continuation of the process of yolk-protein formation which occurs as the egg ripens in situ. The soluble and insoluble proteins show a different pattern of developmental changes, although the major periods of protein synthesis and degradation usually occur simultaneously in both. Analysis of these changes indicates that the soluble proteins in the unfertilized egg are almost entirely yolk proteins, which are consumed before the early prism stage. The insoluble proteins appear to consist mainly of yolk proteins but also include the structural and other functional components of the egg. The periods of yolk-protein breakdown and new protein synthesis show a close correlation with known respiratory changes, and a new interpretation of these changes is given. Periods of intense protein synthesis correspond to those of marked respiratory increase, while intense yolk-protein breakdown takes place at times when the respiration is known to remain more or less constant. It appears from these correlations that the energy for protein synthesis is derived primarily from concomitant oxidations, and that the energy from the hydrolytic degradation of yolk proteins is not stored to any great degree for use in subsequent protein synthesis but is consumed at the time it is generated. There is some evidence for the existence of an undetected pool of aminoacid skeletons or derivatives, possibly an intermediate form through which amino acids pass in the process of protein synthesis.