A chicken tendon explant model system has been developed to investigate the effects of extremely-low-frequency (ELF), low-amplitude, unipolar, square wave pulsed electric fields on fibroplasia in vitro. An electric field parameter set consisting of l-Hz, l-ms duration pulses, with a time-averaged current density of 7 mA/m2 (peak current density 7 A/m2) induced maximal (32 %) increase in fibroblast proliferation in tendon explants exposed for 4 days. Exposure to the same field at an average current density of 1.8 mA/ m2 had no effect on fibroblast proliferation, whereas exposure to current densities on > 10 mA/m2 inhibited proliferation and relative collagen synthesis, without affecting noncollagen protein synthesis. Fibroplasia was significantly increased in explants oriented parallel to applied electric fields having current densities of 3.5 or 7 mA/m2, but there was no detectable effect on explants oriented perpendicular to the same electric field. Fibroblast proliferation and relative collagen synthesis were inversely proportional to donor age for chickens in the 3-to 16-week age group used in this study. For these dependent variables (proliferation and relative collagen synthesis), there was no interaction between donor age and ELF electric field exposure.