A short-term study was made for the urinary mutagenicity caused by cigarette smoking in several volunteers. In order to concentrate the mutagens from urine, cotton linked to a copper-phthalocyanine derivative (blue cotton) was used as an adsorbent. The mutagenicity as measured by the Ames test on Salmonella typhimurium TA98 in the presence of S9 increased rapidly after the start of smoking. When smoking was stopped, the activity began to decrease, and after 6 to 13 hr reached the non-smoking level. There was no great difference among individuals regarding this time-dependent response. These studies have shown that the appearance and disappearance of mutagenicity in urine in relation to cigarette smoking are rapid processes.