Plasma kininogen during the oestrous cycle, early pregnancy and pseudopregnancy in the rat.

1. Changes in the plasma kinin precursor, kininogen, occur following treatment with oestrogens and progestogens. This work was undertaken to attempt to relate plasma kininogen values to hormonal changes in various reproductive states. 2. During the oestrous cycle kininogen concentrations follow a nychthemeral rhythm, kininogen depletion occurring during the increase in general activity of the rat. 3. Superimposed on the nychthemeral changes in kininogen concentration are changes which may be related to the hormonal events of the oestrous cycle. 4. During early pregnancy and pseudopregnancy the kininogen level rises; this increase may be due to changes in the circulating levels of oestrogens and progestogens. 5. It is suggested that the changes in kininogen concentration may result from utilization of kinin which could be of functional significance.