Although it is well documented that children undergo a productive vocabulary spurt late in the second year, it is unclear whether this development is accompanied by equally significant advances in receptive word processing. In the present study, we tested an electrophysiological procedure for assessing receptive word learning in young children, and the impact of productive vocabulary size for performance in this task. We found that 20-month-olds with high productive vocabularies displayed an N400 incongruity effect to violations of trained associations between novel words and pictures, whereas 20-month-olds with low productive vocabularies did not. However, both high and low producers showed an N400 effect for common real words paired with an incongruous object. These findings indicate that there may be substantial differences in receptive fast mapping efficiency between typically developing children who have reached a productive vocabulary spurt and typically developing children who have not yet reached this productive spurt.