We determined the minimum systolic pressures required for blood to enter and begin to displace the plungers of four brands of 3-ml and 5-ml plastic and glass arterial blood gas syringes by combining samples of each syringe size with three sizes of short-beveled needles--25 gauge, 5/8"; 22-gauge, 1"; and 20-gauge, 1 1/2". We measured pressures by use of an arterial-vessel model that incorporated whole human blood and simulated the conditions of a percutaneous arterial puncture. We concluded that all the arterial blood gas syringes that we studied would self-fill in most clinical situations when they were combined with 20- or 22-gauge needles, but that 22-gauge needles with glass syringes were preferable for persons whose systolic pressures were less than 70 mm Hg, for persons in shock, and for persons undergoing CPR. Only glass 3-ml syringes should be used with 25-gauge needles, as two brands of plastic 3-ml syringes with 25-gauge needles required systolic pressures greater than 158 mm Hg and all brands of plastic and glass 5-ml syringes with 25-gauge needles required systolic pressures greater than 247 mm Hg. Because we found no clinical or statistical differences between 20- and 22-gauge needles, we do not recommend use of the larger 20-gauge needle. The syringe-needle combinations that we do recommend should negate the need for, and the potential trauma of, manual aspiration of the syringe plunger.