Summary Experiments on rats, calciphylactically sensitized with dihydrotachysterol (DHT), indicate that challenge with an aluminum dextran chelate (Al-Dex) prior to sensitization causes selective calcium deposition in the vagi, with gastric dilatation and respiratory difficulties. If, under otherwise comparable circumstances, the rats are challenged with Al-Dex after DHT sensitization, there is intense calcification of the analphylactoid shock organs (lips, paws, esophagus, mediastinum and joints), but not of the vagi. The observations show that, by varying the “critical period“between sensitization and challenge, a neurotropic calciphylaxis limited to the vagi, or a calcifying anaphylactoid reaction can be produced with considerable selectivity, at will.