The free amino acid content of 34 hens' oviducts was determined in different phases of the egg formation cycle. In addition, quantitative changes of transmitter-type amino acids (glutamic acid, aspartic acid, taurine, glycine) were monitored in comparison with those of other amino acids participating in protein synthesis (e.g., lysine, tyrosine, valine). Different portions of the hen's oviduct, primarily the magnum and the uterus, were characterised by substantial amino acid secreting activity. The amino acids detectable in the highest quantity were taurine, glutamic acid, glycine and aspartic acid. These amino acids were present in high concentrations throughout the egg formation cycle, but their quantity abruptly decreased at the time of oviposition and ovulation. The quantitative changes of transmitter-type amino acids are mostly independent of those of other amino acids participating in protein synthesis. Since in our earlier in vitro studies the concentration changes of the transmitter amino acids studied were found to affect the motility and agglutination of spermatozoa, they--besides other factors--can be assumed to exert an influence on the sperm storage and release processes of the uterovaginal sperm storage tubules located in the oviduct of birds.