The chemical N-nitrosodimethylamine (NDMA), a suspected byproductof water treatment, was detected by the State of California Department of Health Services (CDHS) in a system receiving water from the Metropolitan Water District of Southern California (MWDSC). As a result, MWDSC initiated several actions including: discussions with the CDHS; collection of samples at various locations in MWDSC's distribution system for NDMA analysis by several laboratories; and, preliminary evaluation of the treatment process and treatment chemicals for the study of NDMA formation. Discussions with the CDHS resulted in a temporary increase in the action level from 2 to 20 ng/L, providing time for the CDHS and water utilities to participate in investigations of NDMA occurrence, treatment chemicals, and treatmentprocesses while still protecting public health. Current significant issues concerning analytical procedures at low ng/L levels still have not been resolved, thus complicating the determination of NDMA occurrence. Preliminary results indicate a possible need for treatment modifications, public disclosure, and other responses by water utilities to the impact of NDMA on their resources. Includes 15 references, tables, figure.