Casey Stoner has returned to testing duties for the Ducati Team in Valencia.
Stoner tested for the Italian team on Monday and will be back on the bike on Wednesday as part of a three-day private test.
Ducati's regular test rider and sometime wildcard entry Michele Pirro, who finished fifth in the most recent MotoGP race in Misano, will perform test duties on the other day.
As well as the Ducati test team, the Aprilia and Marc VDS Honda squads are also on track at the Circuit Ricardo Tormo.
This week's test marks Stoner's third outing of the season for Ducati, following appearances in pre-season testing at Sepang and at a private Barcelona test back in May.
Ducati rider Andrea Dovizioso currently jointly heads the points standings alongside Honda's Marc Marquez heading into this weekend's Aragon race (September 22-24).