A calculation of the probability distributions for the radioactive releases of 131I and 133Xe in a steam generator tube rupture accident is given; the effects of iodine spiking are included. The accident sequence considered involves loss of off-site power after the steam generator tube rupture. Two models are considered for iodine release, one of which yields upper bound releases, and the other of which is our best-estimate model. With the best-estimate model, the probability of a 131I release >2 Ci is 1 × 10−6 per reactor-year. With the upper bound model, the probability of a 131I release >66 Ci is 1 × 10−6 per reactor-year. The probability of a release exceeding 600 Ci of 133Xe is 2 × 10−6 per reactor-year.