To study the duration of inhibition of decidual induction by intrauterine antihistamine, 163 rats were made pseudopregnant immediately after experiencing two consecutive normal oestrous cycles. Decidualization was induced by systemic injection of histamine releaser on 4th day of dioestrous state of pseudopregnancy (L4). Antihistamine (as inhibitor of decidual induction) was instilled unilaterally into uteri of groups of rats, on each of the 14 days (2x4 days of oestrous cycles plus pro-oestrus,

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... oestrus and first 4 days of leucocytic phase of pseudopregnancy). Practically no decidual response was elicited when inhibition treatment was administered during pseudopregnancy; earlier administration resulted in partial inhibition of response.