The experiments reported here have investigated the induction of ethylations to DNA in yeast cells exposed to the chemical mutagen ethylnitrosourea. A similar level of alkylation was seen at the N7 and O6 of guanine and at the N3 of adenine in either log phase cells or in temperature-sensitive cdc4 and cdc7 cells growth arrested at their specific G1 positions. Hence the changes in chromosome structure associated with the above cdc phenotypes do not modify the amount of DNA damage induced by ethylnitrosourea.