Background: Variations in pacing impedance may be observed during implantation of various active fixation pacing leads. However, these variations can be influenced by the nature of the fixation, the implant site, or the measurement method. Here we describe implant dynamics for a 4.1F, catheter-delivered pacemaker lead. Methods: Endocardial active fixation leads were implanted under direct intracardiac visualization in two right atrial sites and three right ventricular sites in isolated swine (n = 6) and human (n = 4) hearts. Impedance measurements were recorded at each site employing three different measurement techniques - Pacing System Analyzer (PSA) 5311, PSA 2090, and the Impedance Tone Box (Medtronic, Inc., Minneapolis, MN, USA) - with four different degrees of lead fixation: helix touching, one turn fixed (1 TF), two turns fixed (2 TF), and overtorqued. Results: Pacing impedances increased from touching to 1 TF to 2 TF at all implant sites in both swine and human hearts. Overtorquing applied to leads was associated with visible distortion at the endocardial tissue-lead interface in at least 60% of swine (18 of 30 implants) and human hearts (nine of 14 implants). Impedance values in the right atrial high septum were significantly larger than in any other implant site (P < 0.05). The three measurement methods did not yield significantly different impedance measurements. Conclusions: Variations in measured impedances were associated with the nature of implant fixation at all sites in both swine and human hearts.