Updated from 1:11 p.m. EDT with new stock prices

Small-cap stocks joined the broader market in opening the second quarter to the upside this April Fool's Tuesday. The Russell 2000 added 3.1% to 709.60, and the S&P SmallCap 600 climbed 3.2% to 376.10.

Among the big gainers was Phoenix, Ariz.-based auto parts retailer CSK Auto ( CAO), shares of which jumped 25.1% to $11.65. O'Reilly Automotive ( ORLY) agreed to buy CSK in a deal worth $1 billion, including about $500 million in debt assumption. Shareholders of CSK will receive $11 in O'Reilly common stock, subject to a collar, plus $1 in cash, for each share of CSK they own. O'Reilly traded up 5% to $29.96.

Shares of Kandi Technologies ( KNDI), a maker of all-terrain vehicles, go-karts and other vehicles in China, zoomed ahead 15.9% to $6.48. For the full year 2007, Kandi announced income of $5 million, or 31 cents a share, vs. $1.1 million, or 10 cents a share, in 2006. Revenue more than doubled to $34.7 million.

Meanwhile, Atlanta-based apparel designer Oxford Industries ( OXM) laced up a 9.8% gain to $24.74 on a SunTrust Robinson Humphrey upgrade to buy from neutral.

Also gaining on an upgrade, Buffalo Wild Wings ( BWLD), a Minneapolis restaurant chain, jumped 12.2% to $27.50 after Morgan Keegan bumped the stock's rating to outperform from market perform.

On the losing side, Arlington, Va.'s MGC Capital ( MCGC) fell 9.5% to $8.23 after Jeffries downgraded the private equity firm to hold from buy.

AAR Corp. ( AIR), an aerospace and defense company based in Wood Dale, Ill., lost 5.1% to $25.89. The Federal Aviation Administration filed a notice regarding maintenance of landing gear on certain Boeing ( BA) airplanes. The FAA note says it is investigating AAR's alleged use of substandard landing gear parts. AAR said that the parts are compliant and noted that the FAA notice does not require taking corrective action.