WHEN the Pacific salmon (Oncorhyncus spp.) heads into fresh water to spawn, the entire generation is doomed to perish on the spawning grounds. In other Salmonidae, however, life does not necessarily terminate at this point and many return to the sea to repeat the reproductive cycle. In Oncorhyncus, the inevitable death is preceded by a variety of metabolic changes, and this metabolic collapse of the Pacific salmon has the characteristics of a predestined bioprogramme. This programme might be the expression of a thanatotropic genetic imprint.