American Michael Phelps, the most decorated Olympian of all-time, will make his return to competitive swimming on Thursday.
Phelps, whose 22 Olympic medals include 18 golds, has not raced since London 2012.
But he is among a star-studded entry for the Arena Grand Prix in Mesa, Arizona, with the 28-year-old Phelps set to be joined by fellow Olympic medallists Ryan Lochte and Katie Ledecky.
The three-day meet has attracted more than 500 swimmers, including almost 50 members of the United States national team.
Phelps has not yet revealed whether he intends to put himself in the selection mix for Rio 2016, and there are many who feel he will struggle to rediscover the form and consistency that made him world swimming's hottest property.
Phelps' four London golds followed an unprecedented eight gold medals in Beijing four years previously - where he broke Mark Spitz's individual record medal haul for a single Games - and six in the Athens Olympics.
He has entered three races in Mesa - 100m freestyle, 100m butterfly and 50m freestyle - which will give an early indication of form and fitness.
Top of his agenda if Phelps has serious competitive intentions this year will be the US national championships in August, which also doubles up as the 2015 world championship trials.
Phelps returned to training last autumn, working throughout the week with coach Bob Bowman at the North Baltimore Aquatic Club.
Speaking recently, Bowman said Phelps was "pretty far" from being back in top form, but backed him to perform competitively in Mesa, starting with Thursday's heats.