Marc Marquez set a scorching pace in the early stages, but dropped down the order during the second half of the 45 minutes.
Jorge Lorenzo's session went in the opposite direction, building speed from a steady start to take over the top spot on the 16th of his 18 laps.
Rear tyre wear appears to be a concern, with many riders already trying the hardest rubber possible.
MotoGP is returning to Argentina for the first time since 1999 and making its race debut at the Termas de Rio Hondo circuit.
Stefan Bradl (LCR Honda), Alvaro Bautista (Gresini Honda), Cal Crutchlow (Tech 3 Yamaha) and Hector Barbera (Avintia) attended a dirty and damp two-day test at the circuit last July.
Crutchlow, now at Ducati, is absent this weekend due to hand injuries sustained in Austin. The Englishman has been replaced by factory test rider Michele Pirro...
1. Jorge Lorenzo
ESP Movistar Yamaha MotoGP (YZR-M1) 1m 42.804s [Lap 16/18] 322km/h
2. Aleix Espargaro
ESP NGM Forward Racing (Forward Yamaha) 1m 43.285s +0.481s [12/13] 319km/h
3. Alvaro Bautista
ESP Go&Fun Honda Gresini (RC213V) 1m 43.637s +0.833s [16/19] 325km/h
4. Andrea Iannone
ITA Pramac Racing (Desmosedici) 1m 43.706s +0.902s [13/16] 322km/h
5. Bradley Smith
GBR Monster Yamaha Tech 3 (YZR-M1) 1m 43.806s +1.002s [17/18] 322km/h
6. Pol Espargaro
ESP Monster Yamaha Tech 3 (YZR-M1)* 1m 43.961s +1.157s [16/19] 322km/h
7. Stefan Bradl
GER LCR Honda MotoGP (RC213V) 1m 44.055s +1.251s [15/20] 324km/h
8. Andrea Dovizioso
ITA Ducati Team (Desmosedici) 1m 44.096s +1.292s [13/17] 322km/h
9. Nicky Hayden
USA Drive M7 Aspar (RCV1000R) 1m 44.333s +1.529s [17/18] 309km/h
10. Valentino Rossi
ITA Movistar Yamaha MotoGP (YZR-M1) 1m 44.502s +1.698s [14/18] 321km/h
11. Scott Redding
GBR Go&Fun Honda Gresini (RCV1000R)* 1m 44.763s +1.959s [13/18] 307km/h
12. Michele Pirro
ITA Ducati Team (Desmosedici) 1m 44.790s +1.986s [16/17] 316km/h
13. Yonny Hernandez
COL Pramac Racing (Desmosedici) 1m 44.889s +2.085s [11/16] 318km/h
14. Marc Marquez
ESP Repsol Honda Team (RC213V) 1m 45.103s +2.299s [3/16] 320km/h
15. Hiroshi Aoyama
JPN Drive M7 Aspar (RCV1000R) 1m 45.137s +2.333s [20/20] 311km/h
16. Dani Pedrosa
ESP Repsol Honda Team (RC213V) 1m 45.724s +2.920s [14/17] 323km/h
17. Colin Edwards
USA NGM Forward Racing (Forward Yamaha) 1m 45.808s +3.004s [15/17] 313km/h
18. Danilo Petrucci
ITA IodaRacing Project (ART) 1m 46.008s +3.204s [10/16] 305km/h
19. Michael Laverty
GBR Paul Bird Motorsport (PBM-ART) 1m 46.167s +3.363s [12/14] 308km/h
20. Mike Di Meglio
FRA Avintia Racing (Avintia)* 1m 46.264s +3.460s [9/19] 307km/h
21. Broc Parkes
AUS Paul Bird Motorsport (PBM-ART)* 1m 46.896s +4.092s [14/16] 303km/h
22. Hector Barbera
ESP Avintia Racing (Avintia) 1m 47.027s +4.223s [14/17] 309km/h
23. Karel Abraham
CZE Cardion AB Motoracing (RCV1000R) 1m 47.037s +4.233s [11/16] 310km/h
Blue Name = Factory - Control ECU hardware, unique manufacturer software.
20 litres of race fuel, 5 engine changes for the season. All engines identical throughout the year within the same team (development freeze).
Black Name = Factory (with concessions) - Control ECU hardware, unique manufacturer software.
Applicable to Factory entries by a manufacturer that did not achieve a dry win in 2013 (only Ducati) plus any new MotoGP manufacturers.
Same 24 litres of race fuel, 12 engine changes, softer rear tyre, no engine development freeze and extra testing opportunities as the Open Category. Fuel to be reduced to 22 litres should Ducati achieve a race win, two second places or three (dry) podiums. If Ducati claims three wins they will also lose the softer rear tyre. All other concessions confirmed until start of single ECU era in 2016.
Red Name = Open - Full control ECU system, hardware and software.
24 litres of race fuel, 12 engine changes, softer rear tyre, no engine development freeze and greater testing opportunities relative to Factory.
Fastest lap at July 2013 test:
GER Honda 1m 44.188s
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