IF you take part in today's International Challenge Day, you will be in good company. Many of Hongkong's celebrities are giving 15 minutes of their time to help Hongkong beat Vancouver, as well as to promote a healthy lifestyle and keep themselves fit. The Governor, Mr Chris Patten, has a busy schedule but he will find time to play tennis, weather permitting, and the organisers are certain to have asked him to register. Legco members who attend today's sitting will take part in the Urban Council's Workercises, a programme of 16 simple exercises that can be done in the office, under the supervision of a qualified instructor before their meeting begins. In the financial sector, members of the Hongkong Gold and Silver Exchange Society have pledged their participation. Mr Thomas Kwok, the manager of the exchange, said that members of the 83-year-old society maintained a healthy tradition of doing lunch-time exercise every day. The Futures Exchange might consider it has done its bit for Hongkong by allowing an aerobics team on to the floor of the exchange earlier this month to help promote the Challenge Day. However, nearly 100 floor traders have agreed to take part. Mr Calvin Ng, executive secretary of the Hongkong Sports Association for the Physically Disabled, said that he had sent registration forms to all the members of his association. ''Some will take part in our regular Wednesday night training at the Hongkong Sports Institute and Wan Chai swimming pool, but it is not possible to say how many of our members have registered,'' he said. ''Some will be doing exercise at home and, because many of our members are junior members, they will take exercise at their schools.'' The Education Department has notified all schools, but the competition has come at an unfortunate time. Most schools are in the middle of internal exams. This has made it difficult to co-ordinate any exercise period for all students, although many of them will exercise as a method of relaxing after the exams are over. Members of sporting clubs are among those who have returned registration forms and, in some cases, it has not been necessary to make special arrangements for exercise. ''Because of our regular activities, such as the tennis league, it has not been possible to introduce any special exercise activity for all our members,'' said Ms Jill Gould, sports co-ordinator of the Ladies' Recreation Club. ''However, members who come to play sport today will be encouraged to ring through to register.'' The armed forces in Hongkong need no encouragement, as exercise is part of their routine training. On this occasion, physical training instructors of all units in the territory will be recording numbers of servicemen and their dependents who are taking part in special training or daily sport, and passing the information on to the co-ordinator at Central and Western District. ''Each unit can arrange what activities it likes,'' said Major Jim Noble, staff officer, physical training, at Headquarters British Forces. ''The headquarters' staff will be participating. I will be circuit training, and others will be playing tennis or carrying out daily dragon boat team training.'' The Hongkong Sports Institute is involved in the torch run at the opening ceremony, and many of its scholars and renowned sportsmen and women will be encouraging people to take part during the day. ''We will be doing anything we can to encourage activity: it is exactly what we should be doing,'' said Mr Paul Brettel, the institute's chief executive officer. ''Anyone taking part in sporting activities at the institute will be encouraged to register.''