Braces from Caitlin Foord and Beth Mead helped Arsenal ease past Women's Champions League holders Lyon 5-1 in Group C action on Wednesday night.
A pair of early goals from Foord and Frida Maanum staked Arsenal to an early 2-0 advantage against the hosts, who hit back through Melvine Malard in the 27th minute to halve Arsenal's lead.
Mead added a goal from a free kick in first-half stoppage time to restore Arsenal's two-goal cushion going into the break.
Foord added her second in the 67th minute to pad the lead and Mead scored again soon after to put Arsenal on top of their group after the first day of matches in UWCL action.
Last season's Champions League runners-up Barcelona kicked off their campaign in style with a thumping 9-0 win over Portuguese champions Benfica in Group D.
Patri Guijarro opened the scoring inside the first minute and Barca did not take their foot off the gas in front of over 5,000 fans at the Johan Cruyff stadium until the last minute.
Forwards Asisat Oshoala and Geyse both registered braces, and there were also goals for Aitana Bonmati, Mariona Caldentey, Ana-Maria Crnogorcevic and Claudia Pina in what was a relentless performance from the home side.
Barca didn't even need to bring on Caroline Graham Hansen or world record signing Keira Walsh, who were both unused substitutes, while Alexia Putellas, who celebrated her second Ballon d'Or win with fans before the game, remains sidelined with a long-term injury.
Jonatan Giraldez's side have now won all five games in all competitions this season, scoring 25 goals and conceding just one. Benfica, meanwhile, remain unbeaten domestically but now face an uphill battle to progress in the Champions League.
In Wednesday's earlier games, Bayern Munich beat Rosengard 2-1 at home with goals from Carolin Simon and Linda Dallmann, while second-half strikes from Valentina Cernoia and Barbara Bonansea helped Juventus to a 2-0 victory away to FC Zurich.