WRK-1, a cloned cell line derived from a rat mammary tumour, responds to physiological concentrations of vasopressin and pharmacological concentrations of oxytocin with increased incorporation of [14C]acetate into lipids and increased protein accumulation. The presence of pharmacological concentrations of insulin, which itself is active on the WRK-1 cells, further enhances the effects of the neurohypophysial hormones. Unlike the action of vasopressin on other responsive tissues, the stimulation of acetate incorporation by WRK-1 cells is not observed until 24 h after the addition of the hormone. The lipids synthesized in response to the hormones are predominantly polar lipids, rather than the triclyclycerold characteristic of the differentiated mammary gland. [1-Deaminocysteine, 8-D-arginine] vasopressin, a vasopressin analogue that lacks pressor activity, has no effect on WRK-1 cells.