(1) The effect of PMS injected subcutaneously on the 12th or 13th day of the last oestrous cycle before service in successive years was studied in Iceland ewes in a series of three experiments, lasting for 6, 5 and 4 years. In each experiment two groups of twenty ewes were used, a PMS group and a control group, and in Experiment No. 2 an extra group of twenty ewes was used in the 1st year. Three dosage levels of 750 i.u., 500 i.u. and 250 i.u. PMS were tried. (2) Multiple births were significantly increased by 750 i.u. and 500 i.u., whilst 250 i.u. appeared to have no effect. The incidence of too many lambs per birth was too high for practical purposes when 750 i.u. were used, 18.4% of the treated ewes having four or more lambs. (3) Repeated treatment with PMS in successive years had no inhibiting effect on superfecundity. (4) Results from field trials carried out on several farms concurrently with the experiments were comparable with the experimental results. The average lambing percentage at birth was 178 for ewes receiving 500 i.u. PMS as compared with 116 for the control ewes.