Thyroids were collected from rabbits on and near the Idaho National Engineering Laboratory (INEL) Site in southeastern Idaho and analyzed for 129I and stable iodine, 127I. The 129I/127I atom ratios in rabbits collected on the INEL site were larger than ratios in rabbits from a control area. Maximum 129I/127I atom ratios (9.1 x 10(-4)) occurred near the Idaho Chemical Processing Plant (ICPP). Based on rabbit thyroid concentrations, the 129I appeared to be deposited primarily in the direction of the predominant winds from the ICPP, northeast and southwest. Dose rates from the 129I to thyroids of INEL rabbits varied from 0.1 to 260 muGy/y (0.01 to 26 mrad/y). Iodine-129 atmospheric releases from the ICPP appeared to have increased the 129I/127I atom ratios on and near the INEL site.