Bioavailability of residues: current status.

1. Bioavailability studies and current methodologies provide a very useful system for assessing the safety of radioactive drug related residues in food-producing animals. 2. Two year chronic toxicity studies provide useful information regarding the toxic properties of the parent drug but they have limited value for the assessment of safety for "second-pass" exposure of radioactive drug related residues of food-producing animals. The relevancy of the traditional two year toxicity study is seriously questioned as a method to determine the safety of drug related residues on the basis of a "second-pass" exposure. 3. More relevant toxicologic information can be gained by using the food-producing animal target species in toxicology studies. Comparative pharmacologic and toxicologic differences between laboratory animals and food-producing animals would be reduced.