Summary. Twenty-six female brown marsupial mice in a laboratory colony were mated at intervals ranging from 1 to 20 days between coitus and ovulation. The numbers of corpora lutea and normal embryos were counted.
A multiple regression model examined the parabolic relationship between the proportion of normal embryos and the time from coitus to ovulation. The proportion of normal embryos increased until a mean of 9·5 days and decreased thereafter. This relationship was independent of the year of breeding and the number of corpora lutea. After survival of spermatozoa for up to 13 days in the female reproductive tract, the fertility levels of females was 88–92%. Low fertility levels after 13 days appeared to be due to a decrease in the number of spermatozoa.
Reproductive tracts from 7 females killed after insemination and examined histologically showed many spermatozoa in the isthmus of the oviduct and the uterus at 5 days post coitum; spermatozoa confined to the isthmus between 6 and 13 days; and few spermatozoa in the isthmus at 14 days after copulation.
A comparison between the fertility levels in the females which had been inseminated once and a further 17 females which had been inseminated 2 or 3 times suggested that spermatozoa from 2nd and 3rd inseminations can contribute spermatozoa for fertilization. In these females fertility levels did not decline with time after the first mating.