Jorge Lorenzo was quickest as the MotoGP field explored Argentina's Termas de Rio Hondo track for the first time.
Friday's first session began half an hour late due to organisational delays ahead of the preceding Moto3 practice.
When the riders did leave the pitlane, it was world championship leader Marc Marquez who set the initial pace.
Visibly more bold than anyone else on the very dirty track, the reigning champion was 1.8 seconds quicker than the opposition at first.
As everyone else got up to speed, times shuffled repeatedly, with both Espargaro brothers, Andrea Dovizioso and Stefan Bradl among the others to spend time in first before Lorenzo's Yamaha hit the front in the closing minutes.
Lorenzo's time of 1m42.804s put him on top by half a second over Aleix Espargaro on Forward's Open class Yamaha.
Gresini Honda rider Alvaro Bautista, one of the riders who tried the track during a promotional visit to Argentina last July, was third, ahead of Andrea Iannone and Bradley Smith.
Marquez ended up 14th, with Honda team-mate Dani Pedrosa 16th. Valentino Rossi was 10th.
Paul Bird rider Michael Laverty had the session's only incident, with a minor crash at Turn 5.
Pos Rider Team Time Gap Laps
1. Jorge Lorenzo Yamaha 1m42.804s 18
2. Aleix Espargaro Forward Yamaha 1m43.285s +0.481s 13
3. Alvaro Bautista Gresini Honda 1m43.637s +0.833s 19
4. Andrea Iannone Pramac Ducati 1m43.706s +0.902s 16
5. Bradley Smith Tech 3 Yamaha 1m43.806s +1.002s 18
6. Pol Espargaro Tech 3 Yamaha 1m43.961s +1.157s 19
7. Stefan Bradl LCR Honda 1m44.055s +1.251s 20
8. Andrea Dovizioso Ducati 1m44.096s +1.292s 17
9. Nicky Hayden Aspar Honda 1m44.333s +1.529s 18
10. Valentino Rossi Yamaha 1m44.502s +1.698s 18
11. Scott Redding Gresini Honda 1m44.763s +1.959s 18
12. Michele Pirro Ducati 1m44.790s +1.986s 17
13. Yonny Hernandez Pramac Ducati 1m44.889s +2.085s 16
14. Marc Marquez Honda 1m45.103s +2.299s 16
15. Hiroshi Aoyama Aspar Honda 1m45.137s +2.333s 20
16. Dani Pedrosa Honda 1m45.724s +2.920s 17
17. Colin Edwards Forward Yamaha 1m45.808s +3.004s 17
18. Danilo Petrucci Ioda Aprilia 1m46.008s +3.204s 16
19. Michael Laverty PBM Aprilia 1m46.167s +3.363s 14
20. Mike di Meglio Avintia FTR-Kawasaki 1m46.264s +3.460s 19
21. Broc Parkes PBM Aprilia 1m46.896s +4.092s 16
22. Hector Barbera Avintia FTR-Kawasaki 1m47.027s +4.223s 17
23. Karel Abraham Cardion Honda 1m47.037s +4.233s 16