Rees was 51 not out while Murray Goodwin was unbeaten on 31 as Glamorgan reached lunch on the first day on 108 for one after winning the toss.
Lancashire, who have a one-point lead over Northamptonshire at the top of Division Two having played a game less, made just the one breakthrough this morning with slow left-armer Simon Kerrigan having William Bragg caught by Ashwell Prince for 24, ending a 43-run opening stand with Rees.
Also in Division Two, Hampshire were 68 for one at lunch after opting to bat first against Kent at Canterbury.
Michael Carberry (33 not out) and Liam Dawson (20no) put on an unbroken 37-run second-wicket partnership following the loss of captain Jimmy Adams for eight.
In the only Division One match starting today, defending champions Warwickshire reached the interval on 54 for two against mid-table rivals Nottinghamshire at Edgbaston.
Captain Varun Chopra and Will Porterfield were the men out, both falling for 10, with Ian Westwood (22 not out) and Laurie Evans (six not out) avoiding any more setbacks for the hosts in the run-up to lunch.
- Sports & Recreation
- Murray Goodwin
- Simon Kerrigan
- Ashwell Prince