In this study we show that treatment of MDA-MB231 hormone-independent human breast cancer cells with oxytocin (OT) or with the OT analogue F314 induces significant growth inhibition together with a change in cell phenotype. In MCF7 and T47D human breast cancer cells, OT inhibits oestrogen-induced cell growth. In these same cells, OT administration significantly enhances the inhibitory effect of tamoxifen on cell proliferation. MDA-MB231, MCF7 and T47D cells all express mRNA specific for the OT receptor. These data suggest that it may be possible to inhibit breast cancer growth using OT and OT analogues.