The National Hockey League will begin its second season tonight when 16 of the league's 21 teams commence play in the best-of-five preliminary round of the Stanley Cup playoffs.

The defending-champion New York Islanders are strong favorites to repeat, with Montreal the other likely finalist. Quebec and Chicago are considered to have the best chances for upsets in early competition.

The second game in each series will be played Thursday, then the teams switch sites for Games 3 and possibly 4 on the weekend. Should a fifth contest be necessary, it would be played Tuesday at tonight's locale.

Added interest in this round is provided by confrontations among hated rivals: Minnesota against Boston and Vancouver against Buffalo. Here is a capsule look at each series:

Toronto at New York Islanders -- Toronto fulfills the formality of providing the Islanders with a first victim, a role the Washington Capitals sought without success. The Leafs gained an 8-5 victory on their last visit to Nassau Coliseum Feb. 17, but the Islanders lost only two of 20 games thereafter.

Pittsburgh at St. Louis -- The teams divided four onesided games on a home-ice basis. The Blues stumbled down the stretch after holding first place most of the way and are ripe to be upset, but Pittsburgh is too inconsistent to manage it. The biggest difference between the teams is in killing penalties; St. Louis ranked second, Pittsburgh 21st.

Edmonton at Montreal -- They also split the season series, with the home team winning every game. The Oilers took the last meeting, 9-1, and in Wayne Gretzky they have the NHL's record scorer with 164 points. Nevertheless, they have no chance of winning.

New York Rangers at Los Angeles -- The Kings won three of four in the regular season and figure to do the same here. The Rangers staggered into the playoffs, not clinching a berth until Friday, but they are enthusiastic. New York's principal problem is a dreadful power play, 20th in the league; only Winnipeg was worse.

Vancouver at Buffalo -- A year ago, the Canucks' Tiger Williams decked then Buffalo Coach Scotty Bowman with a swipe of his stick in the third game of the preliminary round and was suspended for the fourth, when the Sabres concluded the series. This year, the teams divided regular-season play with two games tied.

Quebec at Philadelphia -- The Nordiques won the season series, 2-1-1, taking the last two games, including a 4-1 decision in the Spectrum. The outcome of this matchup depends on the referees' assignments. If they call it tight, the Flyers are in trouble, because Quebec has a power play that ranked seventh in the league and was getting better as the season progressed.

Chicago at Calgary -- These teams played three ties during the regular season, with the Black Hawks earning a one-goal decision in the fourth meeting. With Tony Esposito in the nets, Chicago is always a threat. However, Calgary owns the home-ice advantage and the Flames lost fewer home games, five, than any other team in the league.

Minnesota at Boston -- The North Stars never have won a game in Boston Garden, losing 28 and tying seven. The last meeting there resulted in 406 penalty minutes, an NHL record, with 12 players ejected. The verbal war has heated up since, so this could be a series to remember -- or forget, depending on one's hockey philosophy.