1. Plasma amino acids have been determined in healthy untreated women and in those receiving synthetic steroids to suppress ovulation. Both groups were studied early in the cycle when endogenous sex hormone production is low, and again later in the same cycle, when endogenous or exogenous hormones are at high concentrations respectively.

2. In normal women there is a significant decrease in plasma concentrations of serine, glutamate and ornithine, and of total amino acids in the second half of the cycle.

3. At this time those taking oral contraceptives have significant decreases in plasma concentrations of proline, glycine, alanine, valine, leucine and tyrosine, and of total plasma amino acids. In addition plasma glutamate, glycine, isoleucine and tyrosine concentrations are significantly lower than in normal women.

4. In the interval between completing one course of contraceptive pills and commencing the next, total plasma amino acid concentration reverts to normal, but a significant decrease in plasma glycine concentration persists.

5. It is suggested that these changes are due to the influence of endogenous and exogenous sex hormones respectively.

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