This study examined the effect of nitric oxide (NO) on milk transfer in rats. Pups nursed by mothers that received chronic systemic injections of sodium nitroprusside (SNP) weighed significantly less than pups of mothers treated with either saline or N omega-nitro-L-arginine (NNLA). Intracerebroventricular injection of SNP or L-arginine (L-arg) but not NNLA or saline, caused a significant reduction of milk transfer from mother to pups after a 12 h separation period. Systemic oxytocin (OT) injection reversed the effect of central injection of SNP. Furthermore, SNP and L-arg inhibited, whereas NNLA permitted the characteristic milk ejection burst of OT neurones without changing myoepithelial tissue response to systemic OT. These observations suggest that NO may be involved in the regulation of milk ejection bursts and milk transfer.