SUMMARY Using a standard arena technique, the reactions of groups of albino mice were noted in response to traps baited with urine. The response was measured in terms of latency, duration of investigatory visit, total investigation time and number of visits. Because of a high level of variability between individuals, many of the results were not significant, but it was demonstrated that urine contained olfactory attractants and there was a suggestion that it might also contain repellents. Animals showed a positive response to traps baited with urine from the opposite sex and where there was a difference in response to urine produced during the night and urine produced during the day, 'night' urine generally elicited a stronger response than 'day' urine. Males reacted negatively to male urine, whereas females were more strongly attracted to female urine than to male urine. The attractiveness of female urine to male mice, as measured by duration of investigatory visit, was increased twofold during pregnancy and threefold during oestrus. Oestrous urine tended to be more effective than dioestrous urine. Female urine stored at −10 °C was more attractive to male mice than fresh female urine. After prepubertal castration, males showed no preference for oestrous urine over dioestrous urine. Both normal and prepubertally castrated males, however, spent less time sniffing urine from spayed females than urine from females in oestrus or from non-oestrous females. There was evidence that intact male mice spent more time investigating female urine odours than did castrated mice, but this did not apply to dioestrous urine.