Self-inflating resuscitators (SIRs) are commonly used to assist ventilation in the newborn. Despite their widespread use, there is limited information available on the functional and physical characteristics of SIRs. We studied five new infant bag-valve units to compare their physical characteristics, including reservoir, accumulator, and dead space volumes (VD), pressure relief mechanism (PRM), inspiratory resistance (RI), and functional mechanics including stroke volume (SV), fraction of delivered oxygen (FDO2), and cycle frequency at -5 degrees C, 20 degrees C, and 49 degrees C, using a test lung apparatus and experienced respiratory therapists. In addition, deficiencies and features that may render them less than ideal were sought. All SIRs met minimum standards for reservoir and accumulator volumes, RI (< 20 cmH2O/L/sec), SV (20-70 ml), and FDO2 (> 90%) as determined by the Canadian Standards Association (CSA) and the American Society for Testing of Materials (ASTM). However, the following deficiencies were noted: 1) MPR and Pulmanex had no PRM (ASTM requirement); 2) MPR could not cycle at -5 degrees C; 3) the VDs for Pulmanex (10 ml) and MPR (15 ml) were high; 4) the CPR connector broke during normal use, and the MPR connector frequently disconnected; 5) the Laerdal expiratory valve was incompetent; and 6) the Laerdal and CPR could be misassembled for use. Our study shows that some commercially available SIRs do not meet all CSA and ASTM standards and may not be ideal in specific clinical situations. The choice of a SIR may be dictated by the clinical situation in which its use is intended.