The S&P 500 fell 1.63% to 1,841.67 in afternoon trading Thursday.
The Dow Jones Industrial Average was off 1.22% to 16,236.14, while the Nasdaq was down 2.77% to 4,068.31.
The yield on a 10-year Treasury bond slipped 1.86% to 2.63%.
The price of gold rose 1% to $1,318.90 per ounce.
Irvine-based drug maker Allergan Inc. was down about 3% to $117.40 for a market value of $35.04 billion. The company's rating from Zacks Investment Research was lowered from "outperform" to "neutral" earlier in the week.
Irvine disk drive maker Western Digital Corp. was down about 2% to $89.24 for a market value of $21.08 billion. Competitor Seagate Technology Inc. of Cupertino was also trading down about 2% to a market value of $17.89 billion.
Newport Beach-based DynTek Inc. shares were off about 33% to $10 for a market value of $21.25 million. The technology service provider, which is traded over the counter, was down on unusually high volume Thursday.