The Feulgcn-DNA content of sperm ceUs from 5 bulls was studied by means of microspectrophotomctry after storage at 5°C for 2, 3, 5, and l0 days in a yolk-citrate diluent permitting slow aerobic metabolism. A subsample of sperm cells from each bull was subjected to the Feulgen technique on each of the storage days selected. The cells sampled on each of these days received a standard 12 minute, 60°C hydrolysis. Absorption measurements at 546 m# of the individual cells indicated a marked progressive decrease in the Feulgen-DNA content of the stored spermatozoa. The loss of 30 per cent of the initial DNA at the end of 5 days' storage was highly significant statistically. This decrease approximately parallels the known decrease in fertility of stored sperm cells, as well as the increase in apparent embryonic mortality resulting from the use of similarly aged spermatozoa for artificial insemination.The relative constancy of the deoxyribonucleic acid (DNA) in the resting cell nucleus is a basic tenet of modern biology (8). This tenet for the male gametes is implicit in the recommendation that Feulgen-stained bull spermatozoa could provide a standard reference material for the microphotometric quantitation of DNA in other cell nuclei (14).The assumption of constancy in spermatozoa arises from a number of observations. Among these is the correlation, first established in the bovine (34), between the DNA content of haploid spermatozoan nuclei and the DNA content and the number of chromosome sets to be found in spermatogenic and somatic tissues (10,17,21,34).A second supporting fact is that in the absence of chromosome change the DNA content of cell nuclei appears to be relatively stable (1), for, except in the case of some secretory cells (16), it does not become metabolically so involved in the activity of cells as do other cellular components. This fact has lent support to the commonly held belief that the spermatozoon serves primarily as a convenient mechanism for transport of genetic information via DNA. During this transport it has been assumed that the DNA is wholly passive and is not involved in energy exchange mechanisms.No one seems to have seriously challenged the concept of constancy of the DNA in living spermatozoa, except Salisbury, who did so recently (22) on the basis of an increase in apparent early embryonic mortality in cattle resulting from the aging of spermatozoa by storage at 5°C before insemination (23). The evidence involving the aging of spermatozoa in the incidence of early embryonic mortality in cattle is supported by evidence on aging of spermatozoa in the female reproductive tract for other mammalian species--the rat and the guinea pig (28, 29)--and for poultry (9,20)