Fasting plasma amino acids have been measured in two normal females. In one subject changes in the levels of the amino acids have been followed weekly during four normal menstrual cycles. In the second subject changes in amino acid levels have been followed through a normal cycle, conception and early pregnancy. During a normal menstrual cycle it has been found that not all amino acids behave in a similar fashion. At least three distinct patterns of behaviour in respect of changes of concentration have been identified. In pregnancy it has been found that the depression in amino acid levels, reported by other workers in later pregnancy, in fact occur immediately following conception. The changes in levels of plasma amino acids which occur during the menstrual cycle and in pregnancy appear to be inversely related to the reported changes in progesterone levels which occur during these physiological events.