Pakistan lost openers Ahmed Shahzad (4) and Khurram Manzoor (3) early to Lankan bowler Dhammika Prasad.
Spinner Herath grabbed the third Pakistani wicket of Azhar Ali for Sri Lanka. Herath bowled Azhar when he had made 30 and Younis Khan was batting at the individual score of 18.
Captain Misbah joined Younis after Azhar was returned to the pavilion.
Pakistan are playing the same team that played in the last Test in Sharjah. Mathews wanted to bat first as well but is hopeful that the early moisture might help. Ajantha Mendis is out of the team that played at the SSC, Prasad and Eranga have come in.
This will be their latest bout after the two teams leveled the test series in Sharjah six months ago.
Pakistan will look to end an eight-year drought without a Test win in Sri Lanka while the hosts after being surprised by South Africa on home turf will try to gain the momentum back.
Pakistan, who had ascend to third place in the recent ICC Test rankings, will drop to sixth if they lose the series.
Sri Lanka have a good record at Galle with 11 wins out of 20 matches.
Pakistan: Khurram Manzoor, Ahmed Shehzad, Azhar Ali, Younis Khan, Misbah-ul-Haq (Capt), Asad Shafiq, Sarfraz Ahmed (WK), Abdur Rehman, Mohammad Talha, Saeed Ajmal, Junaid Khan
Sri Lanka: WU Tharanga, JK Silva, Kumar Sangakkara, Mahela Jayawardene, AD Mathews (Capt), KDK Vithanage, N Dickwella (WK), MDK Perera, KTGD Prasad, HMRKB Herath, RMS Eranga