We numerically assess the effects of head properties (anatomy and dielectric parameters) on the performance of a scalp-implantable antenna for telemetry in the Medical Implant Communications Service band (402.0–405.0 MHz). Safety issues and performance (resonance, radiation) are analyzed for an experimentally validated implantable antenna (volume of 203.6 mm3), considering five head models (3- and 5-layer spherical, 6-, 10-, and 13-tissue anatomical) and seven scenarios (variations ± 20% in the reference permittivity and conductivity values). Simulations are carried out at 403.5 MHz using the finite-difference time-domain method. Anatomy of the head model around the implantation site is found to mainly affect antenna performance, whereas overall tissue anatomy and dielectric parameters are less significant. Compared to the reference dielectric parameter scenario within the 3-layer spherical head, maximum variations of −19.9%, +3.7%, −55.1%, and −39.2% are computed in the maximum allowable net input power imposed by the IEEE Std C95.1-1999 and Std C95.1-2005 safety guidelines, return loss, and maximum far-field gain, respectively. Compliance with the recent IEEE Std C95.1-2005 is found to be almost insensitive to head properties, in contrast with IEEE Std C95.1-1999. Taking tissue property uncertainties into account is highlighted as crucial for implantable antenna design and performance assessment. Bioelectromagnetics 34:167–179, 2013. © 2012 Wiley Periodicals, Inc.