Pseudopregnant rats were used to study cytological changes in the endometrium following mechanical or chemical (histamine) stimulation of the mucosa. Normal deciduomata, and decidual growths suppressed by topical intra-lumen application of the anti-histamine drug Benadryl, were examined at intervals from 1 to 216 hr after mucosal stimulation. Results showed no mitotic activity in either the normal or suppressed horn for the first 18 hr followed by diploid activity from 18½ to 27 hr. After 27 hr, the suppressed horn showed no more mitoses, and no development of polyploidy; the normal deciduoma showed diploid mitoses for a longer period, and then the development of polyploidy. In the developing deciduoma polyploidy appeared 36 hr after stimulation. There was an increase in the amount of polyploid cells with the continued development of the deciduoma. It is concluded that ( a ) the suppression of deciduomata by anti-histamine is effected by the suppression of development of the deciduoma and not by a gross destruction of tissue; ( b ) polyploidy develops in the deciduoma, possibly involving all the decidual cells; ( c ) the extensive existence of polyploid cells which cannot reproduce normally or persist is a possible basis for the limited life of the deciduoma.