PIP: Endometrial biopsies at varying times in the menstrual cycle were obtained from 30 healthy women (16-41 years) to be used as controls, and 19 women (16-36 years), who wore an IUD (Mazlin spring, Lippes loop, or Birnburg bow) for from 5 months to 4 years, in order to study histological changes in the endometrium in the presence of the IUD. The biopsy specimen was divided into 2 parts, one of which was frozen, sectioned and stained for alkaline and acid phosphatases, employing Gomori's procedure. The other specimen was fixed overnight, sectioned and stained for mucosubstances with the periodic acid-Schiff method, Alcian blue-periodic acid-Schiff method after diastase digestion, aldehyde fuchsin-Alcian blue sequence, and the high iron diamine-Alcian blue sequence. In addition sections were stained with hematoxylin and eosin for dating and notation of histologic changes. The alkaline phosphatase and acid phosphatase enzymes showed definite cyclic activity in the control group and the presence of the IUD caused only minor alterations in the cycle. The mucosubstances also showed a definite predictable cycle in the normal control group. The study group with the IUD in place showed a similar cycle but the amount of acid mucosubstance was increased throughout all phases of the cycle. In both groups the sulfomucins predominated during the preovulatory or proliferative phase and the carboxymucins predominated during the postovulatory or secretory phase. Certain histologic changes rendered by the presence of the IUD were also evident in this study, 8 patients showing some evidence of mil d to moderate inflammation. The findings support the popular theory that the IUD produces endometrial and intrauterine fluid changes that discourages implantation. A discussion of this presentation is included. It was suggested that a quantitative study of acid phosphatase levels would be relevant in understanding the action of the IUD. Extensive investigation of the lysosomal system will supplement the reported data. Another point was raised concerning the advisability of comparing effects of a variety of IUD designs and also the conclusion that failure of implantation is the cause of reduced fertility in the IUD wearer.