1. The plasma of non-pregnant women contains practically no kinins and only traces of kallikrein. Treatment of the plasma with kaolin produces additional amounts of kallikrein from prekallikrein and of kinin from kininogen.
2. During the second stage of labour, plasma kallikrein is increased and the amount of kallikrein inhibitors is decreased. The plasma kinin-forming activity is only slightly increased due, apparently, to a fall in plasma kininogen.
3. Plasma kallikrein causes a rise in the contractile tonus of the isolated rat and human myometrium.