Margin debt rose further in September, according to the New York Stock Exchange, continuing to rebound along with the equities market from the lowest levels in years early this year.

At the end of September, margin debt was up 6.8% from August to $220.79 billion, according to Big Board data for customers of NYSE-member securities firms. The figure got as low as $173.3 billion at the end of February, the smallest level since the end of 2003.

Market analysts track margin-debt activity as an indication of investors' appetite for speculative trading.

A potential pitfall for those trading "on margin" is a sharp decline in stock prices, which can expose investors to margin calls, requiring them to post additional collateral lest their brokers sell their securities to cover the debt. A wave of margin calls can worsen selling pressure on stocks and was seen as partly to blame for the market's woes last fall and winter.

-By Kevin Kingsbury, Dow Jones Newswires; 212-416-2354; kevin.kingsbury@dowjones.com