Neil Lennon made six changes to the Hoops team which defeated Aberdeen 5-2 as some fringe players got their chance.
Tim Clancy shot the hosts ahead after nine minutes from a Steven Anderson corner and the Scottish Cup finalists retained their lead until half-time.
However, Players' Player of the Year and Celtic top scorer Kris Commons came on as a half-time substitute and levelled from the spot on 53 minutes after Anderson felled Scott Brown in the box.
Further goals from Teemu Pukki on 73 minutes and Virgil van Dijk on 77 minutes seemingly had the Hoops in control, however a Saints backlash was stirring.
St Johnstone's Scott Brown reduced the deficit to 3-2 and, four minutes from full-time, Michael O'Halloran salvaged a point for Tommy Wright's men.