Adult male bobwhite quail Colinus virginianus were fed toxaphene (chlorinated camphene, 67–69% chlorine) at 10 and 50 ppm or endrin (1,2,3,4,10,10-hexachloro-6,7-epoxy-1,4,4a,5,6,7,8,8a-octahydro-1,4-endo-endo,5,8-dimethanonaphalene) at 0·1 and 1·0 ppm and their performance on non-spatial discrimination reversal tasks was measured. The birds were on dosage for 138 days (beginning at the age of 3 days) prior to testing. Two tests (with different pairs of patterns) were conducted with toxaphene-treated birds and five with endrin-treated birds. The toxaphene-treated birds made 50% more errors than did controls (p < 0·02). There was no difference between the effects of the two treatment levels. The performance of the treated birds on a second test equalled that of the controls, indicating that the birds were able to adjust to the pesticide whilst on treatment. Endrin-treated birds made from 36 to 139% more errors than did controls (p < 0·025). The difference between the number of errors made by the controls and the number made by the treated birds on the acquisition, or initial problem of each test, increased exponentially over the first four tests. The 0·1 ppm birds made significantly more errors than the 1·0 ppm birds after reversal 3 or 4 in the first three tests. The endrin effects were reversed after 50 days of untreated feed. The principal effect of endrin was to impair the birds' ability to solve a novel problem. The effects of toxaphene in birds treated as adults appeared after about 30 days of treatment and those of endrin after about 40 days of treatment. Mean brain residues in endrin-treated birds were 0·075 ppm (wet weight basis) for the 0·1 ppm level birds and 0·35 ppm for the 1·0 ppm level birds.