The primary aim of the study was to investigate whether the modified jacket (survival garment 39; Ö39), with substantially increased mass (5.7 kg vs 2.9 kg in the standard jacket (STA)), to be introduced to Gripen 39 pilots on international assignments induces discomfort and/or a too great inspiratory work of breathing during increased gravitoinertial (G) load. The secondary aim was to investigate the effects on work of breathing and breathing comfort of disconnecting the counter-pressure jerkin of the pressure-breathing system. Methods. Ten subjects were investigated in three different conditions (i) wearing STA with counter pressure in the jerkin, (ii) wearing Ö39 with counter pressure in the jerkin (Ö39m) and (iii) wearing Ö39 without counter pressure in the jerkin (Ö39u). The subjects were investigated at increased G-load as follows: 6 G during 15 sec, 6 G during 120 sec, 9 G during 15 sec, alternating 5.5 and 8 G during a maximum of 160 sec. Results. No significant differences were noted between the conditions regarding perceived discomfort, mean arterial pressure, esophageal pressure, elastic inspiratory work of breathing, oxihemoglobin saturation, G-tolerance or G-endurance. The majority of the subjects (8 of 10) preferred Ö39m before Ö39u (p<0.01). Conclusions. Ö39 was well accepted by all subjects at G-loads up to 9G, and did not induce discomfort or affect G-tolerance, G-endurance or work of breathing in comparison with STA. Thus, the present results do not give any reason to discourage further assessments of Ö39 in flight.