Information is presented on some ways in which spermatozoa vary within a mammalian species, and on the relative importance of non-genetic factors affecting this variation. The work is basic to current researches in this laboratory on the genetics of mammalian spermatozoa. Five characteristics of rabbit spermatozoa were examined in permanent nigrosin-eosin preparations: the incidence of stained spermatozoa and the incidence of spermatozoa lacking the acrosomal cap, both related to the fertility of semen; the length, breadth and area of the spermatozoan head. Various factors have been studied that potentially affect these characteristics, with emphasis on variation attributable to technique and observation. The effects of the factors have been recorded in quantitative form and, in the light of the present work, suggestions have been made for the design of future experiments. Experimental and statistical procedures are described. Observations on variation due to biological causes include a separation of the variance between the males of a litter from the variance between the litter means. It has also been shown that the average length of the spermatozoan head in the vas deferens remains virtually unchanged in the ejaculate. Mensuration characteristics (length, breadth and area) are better adapted to objective analysis than are enumeration characteristics (proportions of abnormal spermatozoa).