The session was red-flagged with ten minutes to go following an accident for Valentino Rossi. The Italian was unhurt but his Ducati damaged the airfence, forcing repairs before the session could be restarted.
Alvaro Bautista and Cal Crutchlow also fell this afternoon.
Cardion AB Ducati's Karel Abraham is returning to action this weekend, while Pramac Ducati's Hector Barbera has been ruled out by a broken leg in a training accident.
Barbera has been replaced by former MotoGP race winner Toni Elias, who split from the Aspar Moto2 team after Mugello.
The US GP also sees AMA rider Steve Rapp become the first wild-card entry of the season and first ever for the new CRT class.
Repsol Honda's Dani Pedrosa has one of the new RC213Vs available for Laguna Seca, while team-mate Casey Stoner has the new engine in the present chassis, as he had requested. Ducati are expected to debut 'some' new parts.
for pictures from the grand prix...
1. Dani Pedrosa
ESP Repsol Honda (RC213V) 1m 21.088s
2. Jorge Lorenzo
ESP Yamaha Factory Racing (YZR-M1) 1m 21.422s
3. Casey Stoner
AUS Repsol Honda (RC213V) 1m 21.457s
4. Ben Spies
USA Yamaha Factory Racing (YZR-M1) 1m 21.708s
5. Andrea Dovizioso
ITA Monster Yamaha Tech 3 (YZR-M1) 1m 22.021s
6. Cal Crutchlow
GBR Monster Yamaha Tech 3 (YZR-M1) 1m 22.258s
7. Nicky Hayden
USA Ducati Team (GP12) 1m 22.434s
8. Stefan Bradl
GER LCR Honda MotoGP (RC213V) 1m 22.568s
9. Valentino Rossi
ITA Ducati Team (GP12) 1m 22.772s
10. Alvaro Bautista
ESP San Carlo Honda Gresini (RC213V) 1m 22.853s
11. Aleix Espargaro
ESP Power Electronics Aspar (ART CRT) 1m 23.835s
12. Randy De Puniet
FRA Power Electronics Aspar (ART CRT) 1m 23.870s
13. Colin Edwards
USA NGM Forward Racing (Suter-BMW CRT) 1m 24.732s
14. Toni Elias
ESP Pramac Racing (GP12) 1m 24.739s
15. Yonny Hernandez
COL Avintia Blusens (FTR-Kawasaki CRT) 1m 24.774s
16. Karel Abraham
CZE Cardion AB Motoracing (GP12) 1m 24.849s
17. Mattia Pasini
ITA Speed Master (ART CRT) 1m 25.000s
18. Michele Pirro
ITA San Carlo Honda Gresini (FTR-Honda CRT) 1m 25.065s
19. James Ellison
GBR Paul Bird Motorsport (ART CRT) 1m 25.166s
20. Ivan Silva
ESP Avintia Blusens (FTR-Kawasaki CRT) 1m 25.405s
21. Danilo Petrucci
ITA Came IodaRacing (Ioda-Aprilia CRT) 1m 25.422s
22. Steve Rapp
USA Attack Performance (APR-Kawasaki CRT) 1m 28.016s
Fastest FP1 time:
ESP Honda 1m 21.424s
Official Laguna Seca MotoGP records:
AUS Ducati 1m 20.700s
Fastest race lap:
AUS Ducati 1m 21.376s