40m viewers tuned in to NBC's coverage of swimmer Michael Phelps' pursuit of an eighth gold medal at the Olympics on Saturday, according to Nielsen Media Research data.

The swimming event aired live across much of the US, with NBC averaging 31.1m viewers between 8pm and 11:45 - the network's most watched Saturday night programme since 1990. Those numbers reached their 40m peak between 11pm and 11:30 with the 4x100m medley relay, where Phelps was successful in his bid for the medal.

"The mystery of China combined with the unbelievable phenomenon of Michael Phelps, the terrific performances by gymnasts Nastia Liukin and Shawn Johnson and defending Olympic gold medalists Misty May-Treanor and Kerry Walsh at beach volleyball have captivated the imagination of the country," said Dick Ebersol, chairman of NBC Universal Sports and Olympics, and the executive producer of NBC's coverage of Beijing 2008.

NBC has achieved an average of 30.1m viewers across the whole of primetime for the first week of the summer games, with those American Idol-like numbers representing a 15% improvement on viewership for Athens 2004. It is estimated that 191m Americans have tuned into the networks of NBC Universal during the first nine days of these games, more than the total viewership for the full runs of the Salt Lake and Sydney games.