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Over a period of years interested individuals have submitted many dead or moribund herons of various species to our laboratory to learn whether the birds had been affected by diseases or organochlorine poisoning. Residue concentrations in carcasses of birds and mammals are considered the best measure of sublethal exposure, whereas residues in brains are best to use for diagnosing death by most organochlorine chemicals.... The purpose of the present paper is to document the occurrence and concentration of organochlorine residues in the brains of herons from various areas in the United States. By comparing these residue concentrations with laboratory-determined diagnostic lethal levels, we conclude that some herons were killed by organochlorine poisoning; others were at least seriously endangered by the residues they carried. Complete results of carcass analyses for these and other herons, as well as further details? on residues in brains, will be reported elsewhere. Overall, we analyzed carcasses or brains of more than 70 herons found dead or moribund and 36 others taken in planned collections. Residue levels in carcasses of many herons were not high enough to warrant analysis of brains. In the present paper we compare carcass and brain residues of dieldrin in 23 herons of which both carcass and brain were analyzed.