The U.S. Environmental Protection Agency (EPA) is announcing final regulatory determinations not to issue national primary drinking water regulations for four of the 116 contaminants listed on the Third Contaminant Candidate List. The Safe Drinking Water Act, as amended in 1996, requires the EPA to make regulatory determinations every five years on at least five unregulated contaminants. A regulatory determination is a decision about whether or not to begin the process to propose and promulgate a national primary drinking water regulation for an unregulated contaminant. On October 20, 2014, the agency published its preliminary determinations not to regulate dimethoate, 1,3-dinitrobenzene, terbufos, terbufos sulfone and begin the process to regulate strontium. The agency requested public comment on the determinations, process, rationale and supporting technical information. The agency received comments from 14 individuals or organizations on the preliminary regulatory determinations. After careful review and consideration of the public comments, the agency is making a final determination not to regulate dimethoate, 1,3- dinitrobenzene, terbufos and terbufos sulfone. The agency, however, is delaying the final regulatory determination on strontium in order to consider additional data and decide whether there is a meaningful opportunity for health risk reduction by regulating strontium in drinking water.