General Motors Co. shares rose modestly in their first day of trading Thursday after the car maker's historic initial public offering gave investors their first chance to buy and sell GM stock in more than 18 months.

The shares opened 6% higher as GM Chief Executive Danieal Akerson rang the opening bell on the New York Stock Exchange. In midday trading, the shares were ahead almost 7%.

GM was on pace to sell $18.1 billion in shares in what likely will be the second-largest U.S. initial public offering ever, capping a remarkable two-year turnaround in which the car maker went from begging for a government bailout to posting its first steady profits in more than six years.

The final sales total from the IPO may not be available for several days as the underwriters decide whether to sell the "overallotment" shares that sometimes are offered when IPOs experience high demand.

Buyers of the GM shares included giant pension and hedge funds as well as GM factory workers and retirees. Among foreign buyers was China's largest car maker, SAIC Motor Corp. , which is GM's biggest partner in the world's largest auto market. SAIC said it bought about $500 million of shares for a GM stake of close to 1%.