The Sensex, which had surged to 18,278.62 in opening trade, was down by 149.93 points at 18,052.27 at 1100 hours.
Similarly, the broad-based National Stock Exchange Nifty index was down by 46.40 points at 5,409.60 at 1100 hours.
Brokers said the emergence of profit-taking after a rally during the previous two sessions mainly dragged the Sensex down.
Stocks of state-run BHEL fell by 2.96 per cent to Rs 2,067, while engineering major, Larsen and Toubro was down 1.98 per cent at Rs 1,627.90 as participants locked in gains at prevailing levels.