Royal Mail shares rose 38% to 455p in their first day of conditional dealings on the London Stock Exchange.
The hugely oversubscribed sale was priced at 330p a share, valuing the 500-year-old firm at £3.3bn. At one point, the price hit 459.75p.
Private investors received 227 shares each. In all, more than 225 million shares were traded on Friday.
The shares are listed officially next Tuesday, but City institutions began conditional dealings on Friday.