Free and protein amino acid contents of whole-plant maize ensiled with or without urea were determined at intervals during the fermentation process. The free amino acid content of both untreated and urea-treated silage increased with time in storage, but the increases were generally larger in the treated silage. The true protein of untreated silage decreased with time in storage. In urea-treated silage many individual amino acids increased considerably. Among those were the essential amino acids isoleucine, lysine, threonine and valine. The result was a net gain of several amino acids in the treated silage while there was a general decline of most amino acids in the untreated silage. Urea-treated silage at day 20 contained 38 % more true protein and 150% more free amino acids than did untreated silage. The implications of these changes for dairy cattle nutrition are discussed.