In an experiment conducted during the summers of 1949 and 1950 at the Patuxent Research Refuge, Laurel, Maryland, two aerial applications of DDT were made at the rate of 3 pounds per acre. Sprayings were made over an experimental area of about 20 acres of abandoned fields bordered by woods and hedgerows. T\vo hundred and ninetythree bird boxes, with three different sizes of openings to accommodate various species of hole-nesting birds, were distributed throughout this area and in a similar check area of the same size. House wrens were the most numerous and most productive of the species restricting their activities to the areas under study, and therefore they provided the most reliable information on survival and growth-rate after spraying. A total of 149 nestling wrens were under observation in 1949 and 154 in 1950. .....There were only slight decreases in the total adult bird populations, no increase in the number of cases of desertion, no significant differences in feeding ranges through the reduction of insect food, nor any apparent avoidance of the sprayed area by nesting birds. DDT at the 3 pounds-per-acre dosage caused considerable mortality to young nestlings. The fledging success in six active first-brood wren nests in 1949 was 28 percent compared to 86 percent in the unsprayed check. In 1950, when the area was sprayed at an earlier date in respect to both the calendar and the nesting progress of the first-brood wrens, fledging success in the sprayed area was 70 percent compared to 73 percent in the check. Average weights of first-brood wren nestlings in the sprayed area in both 1949 and 1950 were significantly lower than those in the check area. The weights of second-brood nestlings in 1949 were lover than those of the checks but the differences were not significant statistically. In 1950 the weights of second-brood house wrens in the sprayed area closely approximated those of the check.....Insect-sampling showed that shortly after spraying, the check area contained 17 times as many insects as the sprayed area in 1949 and 6 times as many in 1950. Six weeks elapsed in 1949 and 8 weeks in 1950 before the invertebrate populations in the two areas became similar. Since the density of insects necessary to sustain bird life in any area is unknown, it is uncertain to what extent this reduction in food contributed to the mortality of nestlings in the sprayed area or to their being underweight in comparison with the checks. The blowfly parasite, A paulina sialia (S.&B.),did not occur in sufficient numbers in 1950 to offer evidence as to whether DDT provided any protection to nestlings as was suggested by observations in 1949. The chemical analyses of dead birds collected dwing the course of experimentation show a wide range in quantities of DDT present. For instance, quantities of DDT detected chemically in nestlings thought to have died from lethal dosages of DDT ranged from 2.6 to 77.0 micrograms per gram of body weight, and one set of wrens exhibiting symptoms of DDT poisoning contained 3.8 mcg./g. Runts from four different nests assumed to have died as a result of unsuccessful competition with their larger nestmates, however, were found to contain quantities of DDT ranging from 14.3 to 38.0 mcg./g. It is conceivable that nestlings inadequately fed or for other reasons being low in vitality could succumb to small quantities of DDT, whereas healthier individuals being nourished normally by their parents might survive for a longer period even though they consumed more DDT. Furthermore, it must be kept in mind that the amount of DDT that constitutes a minimum lethal dose for juvenile birds is still unknown. Also, there is no standard for correlating the quantity of DDT found in tissues and the quantity ingested.