The benchmark Korea Composite Stock Price Index fell 2.25 points, or 0.09 percent, to 2,490.25 in the first 15 minutes of trading.
Foreign and retail investors sold a combined 13.4 billion won ($12 million) worth of stocks, offsetting 15 billion won bought by institutions.
On Wednesday, foreigners bought 111.9 billion won worth of stocks, helping the main index end at an all-time high of 2,492.50.
Tech stocks led losses, with market bellwether Samsung Electronics Co. falling 1.08 percent and No. 2 chipmaker SK hynix Inc. shedding 2.08 percent.
Among gainers, Hyundai Motor Co. rose 1.01 percent and leading chemical firm LG Chem Ltd. climbed 1.94 percent.
The local currency was trading at 1,124.75 won against the US dollar, up 3.15 won from the previous session's close. (Yonhap)