The transepithelial potential difference (TEPD) and resistance (TER) in the alveoli, and the pressure in the mammary gland were measured in lactating mice at different times after they had finished suckling their young. Oxytocin administration caused a rise in TEPD, the amplitude of which decreased with time after cessation of the pups feeding. At 2·5 h after cessation, oxytocin administration caused a short-term decrease in TER, but at 5 h there was a two phase reaction: a fall in TER followed by a rise. At 20 h there was a one phase reaction, a rise in TER. TEPD in the mammary gland alveoli increased from its initial value of 18±1 mV to 25±1 mV at 2·5 h, and thereafter fell to zero by 15 h after suckling. The initial value of TER, 114±1 kΩ, increased to 131±4 kΩ at 2·5 h and then decreased by 5 h after suckling, but unlike TEPD it did not reach zero. These experiments show that extending the period after cessation of suckling does not cause a rise in intramammary pressure and that oxytocin injections cause a short-term rise in milk pressure. From these results, we propose a hypothesis of reducing ionic concentration gradients between intracellular fluid, cytoplasm of the secretory cell and milk at extended periods after cessation of suckling, starting at 2·5 h.