Circadian variation in the susceptibility of mouse epidermis to chemical carcinogens.

Groups of hairless nice were given a single topical application of each of two highly-reactive carcinogens (methylnitrosourea and beta-propiolactone) at 12(00) and 00(00). The tumor yield was as follows: for methylnitrosourea there was a slightly higher yield of skin tumours in the animals painted at 12(00); for animals painted with beta-propiolactone, there was a significantly higher skin tumour yield in those painted at 12(00). When the groups painted with the two carcinogens are considered together, there was a significantly higher number of animals with all types of tumours in the group painted at 12(00). Hence, there seems to be a circadian variation in the sensitivity of a tissue to a single contact with a chemical carcinogen. This may be related to the proliferative state of the tissue involved: at 12(00), DNA and cell division takes place at a more rapid rate than at 00(00).