Retail traders booked profits, weighing on Saudi Arabia's index TASI as petrochemical stocks led declines.
The benchmark ended 0.4 percent lower at 6,684 points, declining for second day since Wednesday's 14-week peak. It rose 2.3 percent in April.
"We saw a resurgence in volumes that means the retail investors are heavy, so moves are more volatile," said a Riyadh-based trader who spoke on condition of anonymity. "I would expect that trend to continue."
Analysts said the market must consolidate before it can move higher.
The petrochemicals index slipped 0.6 percent.
Heavyweights Saudi Basic Industries Corp (SABIC) and Rabigh Refining and Petrochemical Co fell 0.5 and 2.1 percent respectively.
Saudi Kayan Petrochemicals dipped 0.8 percent.
UAE bourses fell to a two-week low, as investor concerns on the country's inclusion in MSCI emerging market status weighed.
Dubai's index DFM retreated 0.4 percent lower to 1,628 points, its lowest close since April 14.
Dubai Investment, down 2.8 percent, fell for second session after going ex-dividend.
UAE bourses announced a delay on Thursday in implementing the new payment settlement system - Delivery Versus Payment (DvP), a pre-requisite for MSCI emerging market inclusion ahead of a review in June.
"They [UAE] weren't serious contentions to be involved," says Robert McKinnon, ASAS Capital chief investment officer. "The anticipated impact on the market was exaggerated. I think it's more than just a DvP issue."
Abu Dhabi's index ADI dropped 0.2 percent to 2,691 points, also a two-week low, as bluechips weighed.
Dana Gas fell 1.5 percent and indebted-developer Aldar Properties dropped 0.7 percent.
Abu Dhabi Commercial Bank fell 2.1 percent, declining for a second session since Wednesday's 30-month peak. The lender's shares had gained after its first-quarter profit more than doubled.
Petrochemical stocks weighed on Saudi Arabia's index TASI as investors booked recent gains.
"They've come from a long rally - it's only normal for a breather," said Haissam Arabi, chief executive and fund manager at Gulfmena Alternative Investments.
The benchmark slipped 0.2 percent to 6,697 points, declining for a second day since Wednesday's 14-week high.For all the latest business news from the UAE and Gulf countries, follow us on Twitter and Linkedin, like us on Facebook and subscribe to our YouTube page, which is updated daily.
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