Updated from 11:24 a.m. EDT

Shares of several rental-car companies took it on the chin Monday after Morgan Stanley downgraded the stocks to underweight from equal weight.

The firm cut

Avis Budget Group

(CAR) - Get Report

,

Dollar Thrifty

( DTG) and

Hertz

(HTZ) - Get Report

, saying that travel is down and that fewer cars are being built for rental companies, which will make the cost of buying cars more expensive.

Avis closed down $1.92, or 6.5%, to $2756. Dollar Thrifty was down $2.40, or 5.7%, to $40.06, and Hertz was off 53 cents, or 2.1%, to $25.01.

Zoltek

(ZOLT)

fell 3% after the carbon-fiber maker filed a $350 million mixed shelf registration statement with the

Securities and Exchange Commission

. The mixed shelf will consist of debt, stock, warrants and units.

Selling shareholders may sell up to $30 million of the total. Zoltek plans to use the proceeds from the offering for general corporate purposes, including debt repayment. Shares closed down $1.24 to $40.41.

Shares of the

Blackstone Group

(BX) - Get Report

settled down a day after the firm debuted on the

NYSE

. On Friday, the shares jumped 13% after the company priced its initial public offering. The stock today closed down $2.62, or 7.5%, to $32.44.

Pier 1 Imports

(PIR) - Get Report

rose 3.6% after UBS upgraded shares in the furniture chain to buy from neutral. The firm boosted its price target on the stock to $12 from $7.

UBS also trimmed its loss forecast for the company and now believes that Pier 1 will post a 2007 loss of 75 cents a share, down from its earlier forecast of $1.35 a share. Pier 1 closed up 30 cents to $8.61.

Shares of

Omnicom

(OMC) - Get Report

rose 2.2% after Deutsche Bank upgraded the media services company to buy from hold.

Deutsche said the company is winning new business and that the 2008 Olympics should be a boon. The firm has a $118 price target on the company. Shares closed up $2.19 to $103.97.