The variability in toxic response of Japanese quail (Coturnix coturnix japonica) to a standardized 5-day subacute feeding trial was studied while age was increased at weekly intervals from 1 to 21 days and at different times with 14-day-old birds. The objectives were to identify the strengths and limitations of this subacute toxicity protocol and to provide possible explanations for differences in response among the various classes of pesticides. The variables included the median lethal concentration (LC50), the dose-response slope, food consumption, mortality patterns, and procedures of preparing toxic diets. Carbamate, organophosphate, chlorinated hydrocarbon, and organic mercury compounds were represented. A total of 60 subacute tests were involved in this evaluation. The general conclusions of the study are as follows: (1) The basic protocol yields good within-laboratory reproducibility of results. (2) The LC50s increase as birds grow older, and the change was reasonably predictable between 7 and 21 days of age. (3) Monitoring of time-related response patterns, such as food consumption, onset and remission of overt toxicity, and mortality, is critical to the evaluation of potential hazard. (4) Different orders of toxicity may occur at different ages for an array of compounds, but differences do not appear to be large enough to seriously alter the interpretation. (5) For optimal comparisons, all testing should be conducted on birds of a single age.