Serum ALP levels both during and after the discontinuation of heparin treatment. Rats were injected for 28 days with vehicle alone (•) or unfractionated heparin at concentrations of either 0.5 U/g/day (▪) or 1.0 U/g/day (▴) and then allowed to live for an additional 28 days with no further treatment. During the course of this experiment, tail vein blood samples were collected weekly and assayed for ALP activity as an index of bone formation.