Many changes have been noted in the plasma proteins of women taking oral contraceptives and a similarity between the metabolic effects of oral contraceptive therapy and pregnancy has been observed by Tyler (1964) as well as by Crawford & Rudofsky (1966). The existence ofdiminished serum cholinesterase activity in pregnancy (Schnider, 1965; Robertson, 1966) and in women taking oral contraceptives (Robertson, 1967) is reported. Robertson (1967) has shown electrophoretic changes in serum plasma proteins during oral contraceptive therapy. In the present study, changes are reported in plasma cholinesterase isoenzymes in women taking oral contraceptives all ofwhich contained oestrogen. Thirty-eight plasma samples from healthy young women were examined by electrophoresis in 13\m=.\5%starch gel (Smithies, 1955) using 0\m=.\03m-borate buffer pH 8\m=.\3,and a voltage gradient of 10 V/cm for 3 hr. Erythrocytes, thrice washed with isotonic saline, were disrupted by an ultrasonic disintegrator (MSE type E 7680/3) for about 30 sec in order to free the acetylcholinesterase from the red cell stroma. These sonicates were examined in the above system. After electrophoresis, the gels were stained using a-naphthyl acetate as substrate and the liberated a-naphthol coupled with Fast-Red TR salt. The samples were supplied blind, a certain number of controls (pre-pill patients and patients using an intrauterine device) being included with samples from women taking oral contraceptives. Except for some ambiguity in six cases, it was readily apparent whether or not the sample was a control. Plate 1, Figure 1 illustrates the typical changes which appear to be confined to the faster moving and minor components of the isoenzyme pattern, the two major bands (C4 and C5) being unaffected in intensity. No differences were detected in the effects of different oestrogen/ progestagen combinations. Only the activities of the isoenzymes appear to be affected, no changes in their mobilities were observed. Components Cj, C2 and C3 were affected, with C3 being the most sensitive. The C5+ (in this zymogram, C6 + ) variant which is known to be associated with high plasma cholines¬ terase activity, was not observed (Harris, Hopkinson, Robson & Whittaker, 1963). No changes were observed in erythrocyte esterase patterns as can be seen in PI. 1, Fig. 2. The observed changes appear to manifest themselves with