In a study of different methods of handling stallion semen for deep freezing, ejaculates were divided into 3 portions, the first of which was diluted 1:2 with lactose--egg yolk--glycerol diluent and frozen in pellet form. The second aliquot was centrifuged without any diluent and the third portion was initially diluted with an experimental diluent (Merck) and then centrifuged for 5 min at 1000 g. The second and third portions were frozen in large volume straws each of which contained one whole insemination dose of 1 or 2 X 10(8) progressively motile spermatozoa. The addition of a diluent to the semen before centrifugation and freezing (portion 3) resulted in an increase in sperm motility after thawing. Motility was further increased by the use of a recently developed diluent after centrifugation and before freezing. In one fertility trial, 12 of 19 mares (63%) conceived following a single insemination of frozen semen during one oestrous period.