Oxytocinase activity in uterine fluid, plasma and in both particulate (27000 x g fraction) and soluble fractions (post 27000 x g fraction) of endometrium were determined using S-Benzyl-L-Cysteine-β-Naphthylamide as substrate at various physiological states of the endometrium. It was observed that oxytocinase is 'washed out' from particulate to soluble fraction under the influence of progesterone and this process is inhibited by oestrogen and components of Floraconta (R). Uterine fluid and plasma oxytocinase activities also increase in early pregnancy and the secretory phase and is reduced in the proliferative phase and in cases of uterine dysfunction induced by copper-IUD, lactational amenorrhoea and Floraconta (R) administration. Kinetic study shows that both particulate and soluble oxytocinase have identical K(m) with nearly identical V(max) at any particular phase of the tissue. In in vitro system, progesterone increases V(max) but oestrogen decreases V(max) without affecting K(m). It is concluded that ovarian hormones produce a quantitative alteration in the subfractional distribution of oxytocinase.