Developed as strain MF by Holme and Piechuta (1979) by selective breeding of Sprague-Dawley outbred rats. Individuals were injected sub-cutaneously with egg albumin and B. pertussis vaccine i.p. then challenged with areosolised egg albumin after 14-18 days. Individuals within litters with the most severe symproms (longest duration of dyspnea) were selected and mated brother x sister. Later re-named APR (Apnea Prone Rat).