Abstract: Counter-marking in mice, Mus musculus was investigated by analysing urine deposition on filter paper marked asymmetrically with urine of the opposite sex. Intact males deposited large numbers of urine spots with a marked angular bias towards previously marked quadrants. More spots were deposited on proestrous and ovariectomized donor urine patterns, their distribution being more centrifugal on oestrous urine and more centripetal in quadrants containing a large female urine spot in a central position. In contrast, castrated male mice deposited very few spots with no angular bias. Female urine patterns showed angular bias in response to intact, but not castrated male donor urine, a larger number of spots being produced by oestrous females. Thus the pattern of deposition offers scope for two-way communication of information about reproductive potential.