Shares of Agco ( AG - Get Report) were among the NYSE's losers Friday, falling 7% after the maker and distributor of agricultural equipment slashed its 2006 earnings and revenue forecast.

The company sees earnings of $1 a share, with a 2% to 3% revenue decline. Previously, the company said that earnings would grow by about 10% and that revenue would be slightly below 2005 levels. During 2005, the company posted adjusted earnings of $1.46 a share on revenue of $5.4 billion. Analysts polled by Thomson First Call project 2006 earnings of $1.45 a share on revenue of $5.33 billion. Agco blamed the shortfall on its continued effort to reduce dealer inventories, softening market conditions and thinner-than-expected margins. Shares were trading down $1.96 to $24.29.

General Motors ( GM - Get Report) tumbled 5% after Jerry York, an ally of shareholder Kirk Kerkorian, stepped down from the automaker's board. In addition, Kerkorian's firm backed away from plans to buy more GM shares. The move comes after GM ended talks about an alliance with Nissan and Renault, a deal Kerkorian strongly supported. GM shares were down $1.66 to $31.47.

Guitar Center slid 8% after the music retailer cut its third-quarter forecast. The company estimates earnings of $10.6 million to $11.3 million, or 36 cents to 38 cents a share, and revenue of about $473 million. The guidance includes a one-time real estate gain of 4 cents a share. Previously, the company said that it would earn 40 cents to 46 cents a share, before the real estate gain, on revenue of $489 million to $501 million. Analysts project earnings of 42 cents a share on revenue of $487.7 million.

"We experienced uneven sales in both our Guitar Center and Musician's Friend divisions," the company said. "Our Guitar Center comparable-store sales were soft in the first two months of the quarter, with trends improving markedly in September." Shares were trading down $3.63 to $42.

Shares of Global Signal ( GSL - Get Report) jumped 9% after the cell-phone tower company agreed to be acquired by Crown Castle International ( CCI - Get Report) for about $4 billion in cash and stock. Including the assumption of the debt, the deal is worth about $5.8 billion. The merger, which is expected to close during the first quarter of 2007, will result in cost savings of about $12 million to $15 million annually. Shares of Global Signal were trading up $4.59 to $54.69, while shares of Crown Castle were trading down $1.15 to $33.60.

ScanSource ( SCSC - Get Report) rose 7% after the maker of bar code scanners put first-quarter revenue above analysts' expectations. For the period ended Sept. 30, the company estimates revenue of $487 million to $495 million. Analysts project revenue of $474.7 million. During the year-earlier quarter, the company posted revenue of $390.4 million. Shares were trading up $2.01 to $32.

NYSE volume leaders included GM; Micron Technology ( MU), down $2.31 to $15.23; Lucent Technologies , down 7 cents to $2.29; CVS ( CVS - Get Report), down $2.24 to $28.69; Rite Aid ( RAD), down 20 cents to $4.67; Ford ( F), up 2 cents to $8.35; Sprint Nextel ( S), up 51 cents to $18.37; Walgreen ( WAG), down $2 to $41.39; and Qwest ( CY), up 8 cents to $8.50.

Nasdaq volume leaders included Cisco Systems ( CSCO), up 21 cents to $24.11; Sirius Satellite Radio ( SIRI), down 6 cents to $3.89; Intel ( INTC), down 18 cents to $20.60; Applied Materials ( AMAT), up 28 cents to $18.36; Oracle ( ORCL), up 9 cents to $18.35; Level 3 Communications ( LVLT), down 2 cents to $5.14; JDSU ( JDSU), down 2 cents to $2.18; and Microsoft ( MSFT), down 8 cents to $27.84.