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The crystal structures of two tyrosine dipeptides, glycyl-L-tyrosine dihydrate (I) and N-acetyl-L-tyrosinemethylamide (II) are presented. They crystallize respectively in the space group P21212~ with a= 12.075, b=22.405 and c=4.850 ,~, Z=4 for (I), and in the space group P4x with a=b=8.588 and c=17.221 A, Z=4 for (II). The final reliability indices R were 0-037 and 0.041 respectively. The conformations of the tyrosine residue in both compounds are different. However a general study of that residue in different crystal structures shows that the side chain has only three preferential conformations, with Zx--60, 180 or 300 ° and 2'21 between 60 and 100 ° (average value of 75°). A short comparison with energy calculations is given.