Working memory (WM) is critical for problem solving and reasoning. Beginning in infancy, children show WM capacity increasing with age but there are few validated tests of WM in very young children. Because rapid brain development may increase susceptibility to adverse impacts of prenatal neurotoxicant exposure, such as lead, tests of WM in very young children would help to delineate onset of developmental problems and windows of susceptibility.Our objective was to assess the feasibility of administering a Delayed Spatial Alternation Task (DSAT) to measure WM among 18- and 24-month old children enrolled in an ongoing longitudinal birth cohort study and compare DSAT performance with age and general cognitive development. We further explored whether prenatal lead exposure impacted DSAT performance.We assessed 457 mother-child pairs participating in the Programming Research in Obesity, GRowth, Environment and Social Stressors (PROGRESS) Study in Mexico City. The DSAT and Bayley Scales of Infant Development (BSID-III) were administered at 18- and 24-months. Lead was measured in maternal blood collected during pregnancy (MBPb) and in a subsample of children at 24-months (CBPb). We regressed DSAT measures on MBPb and CBPb, child sex, and maternal age, education, socioeconomic status, and household smoking. We compared DSAT performance to BSID-III performance with adjusted residuals.24-month children perform better on the DSAT than 18-month children; 24-month subjects reached a higher level on the DSAT (3.3 (0.86) vs. 2.4 (0.97), p