Pompey are keen to put a short-term deal in place, with it hoped that they can take the 20-year-old until January.
Mendez-Laing is one of four players to have been placed on the transfer list by Peterborough following a breach of club discipline.
He only linked up with Posh over the summer, with a permanent move made from Wolves.
The youngster previously spent time on loan at London Road, with a season-long spell in 2010/11 offering him regular first team football.
His days at Peterborough would appear to be numbered, though, and Pompey are exploring the possibility of adding him to their ranks in League One.