Two harbour porpoises, a male and a female, both around 10 months old, which had to be left alone for periods between bio-acoustical investigations, developed stereotyped motor patterns and signs of boredom. Therefore, toys of different kinds were introduced to them as occupational therapy. Several kinds of toys were accepted, although some were more preferred than others. A progressively decreased approach time to new toys was observed, ranging from hours in the inexperienced animal to a couple of minutes in the experienced one. The play activity was most intense in the hour before feeding, and one of the animals spent as much as appr. 65% of this hour in play. A female, which during an early isolation developed a “cage habit”, could be partly diverted from this if provided with toys.