Abstract: A pressure responsive delayed release standing heat detector for cows or other livestock comprises a small, single-use, flexible packet to be secured to the body of a cow in a position to be contacted by a mounting animal. A sponge-like body partially presaturated with a colored indicator fluid is contained within an air tight flexible plastic bulb or container having a stopper which is retained by friction and adapted to be expelled when the sponge is compressed under sustained pressure of a mounting animal. Expulsion of a stopper element and discharge of the indicator fluid leaves a telltale indication that the animal has been mounted.