The magnitude of the forces causing closure of the bovine teat canal was estimated by measuring the pressure required to cause retrograde flow of milk into the canal from a manometer attached to the external teat orifice. Before milking, the hydrostatic pressure required to cause retrograde flow was 12 kPa or greater. Retrograde flow occurred at a pressure of 4–6 kPa once the teat canal surface had been wetted. This 2- or 3-fold fall in the closing forces presumably resulted from the combined effects of fracturing the teat canal epithelium and loss of initial resting tension in the smooth muscle fibres surrounding the teat canal. After milking, retrograde flow into any teat canal occurred at a pressure ∼ 1 kPa lower than the wetted, pre-milking value. This small drop, and subsequent recovery ∼ 30 min after milking, reflects the change in tonicity of the smooth muscle fibres which are stretched during milking. At hydrostatic pressures below 7–9 kPa, retrograde flow could be arrested momentarily by the occurrence of a teat contraction. At 4–7 kPa, the wave of contraction was also capable of expelling 4–14 μl milk from the teat canal indicating the outwardly directed, peristaltic nature of the phasic muscular activity.