Haemodynamic instability is a recognised phenomenon in critically ill patients undergoing hyperbaric therapy. Instability may result from the effects of ambient pressure on the cardiovascular system, devices involved in infusion of drugs and fluids, or a combination of the two. The effect of hyperbaric pressure on air-containing spaces in syringes has not been previously measured. We connected 60-ml syringes (Terumo Corporation, Tokyo, Japan) containing coloured water to low volume extensions via three-way taps. We examined the effect of pressurisation to 2.4 and 2.8 atmospheres absolute (ATA) on the syringes by measuring the displacement of the coloured water in the low volume extension set. There was compression of air spaces within the syringe causing retrograde flow of fluid within the low volume extension set. The mean (95% CI) change in volume was 154 (141-168) microl at 2.4 ATA, and 197 (183-212) microl at 2.8 ATA (both p < 0.0001). We conclude that hyperbaric exposure may cause clinically significant changes in syringe function at infusion rates < 100 ml. h(-1).