The effects of daily oxytocin (OT) (2 IU) injection (i.m.) on lactation performance of 25 Mehraban ewes were studied. Eight control ewes were injected with 1ml of saline. Eight ewes were injected with OT after weaning (POT), and nine ewes were injected with OT from day 15 of lactation (WOT). Total milk production for WOT ewes was 55.5 and 24.7% greater (P<0.05) than for control and POT ewes, respectively. POT group produced 24.7% more milk than control group. Lactation length was 175 days for WOT and POT groups, and 143 days for control ewes. Daily milk yield after weaning was greater for WOT and POT as compared with control ewes. WOT lambs had a greater daily weight gain as compared with POT and control lambs. WOT ewes lost more weight during the suckling period, but the difference in ewe live weight loss after weaning was not significant between the experimental groups. Fat content as a percentage of milk dry matter was greater for WOT than for control and POT ewes. Milk density, pH, freezing point, and protein, lactose and ash contents were not affected by OT treatment. Somatic cell count (SCC) was greater for control than for POT and WOT groups and increased as lactation progressed. The results of this experiment support the hypothesis that OT may be involved in mammary cell maintenance and metabolism, in addition to causing myoepithelial cell contraction and milk letdown.