When adult male dogs were placed in pens previously occupied by other dogs, a mean of 24.6 urine marks were deposited in the first 2 hrs; marking frequency was then reduced to about two to five marks per 2 hr over the remainder of the day and on subsequent days. A high rate of marking behavior was again observed when subjects were placed in a special marking pen for 5 min. These observations demonstrate some similarities between urine marking in male dogs and scent marking in males of some rodent species. Following castration no quantitative changes in urine marking in the special marking pen were evident after 5 mo of postcastration testing even though a decline in mating behavior occurred within 2 mo. This contrasts with the rather prompt reduction in scent marking in male rodents following castration.