The late positive potential (LPP) depicts brain electrical activity during both automatic and controlled sustained attentional processing of emotional stimuli. We investigated in a sample of 18 healthy women how the LPP is modulated by facial expression during an explicit valence rating task and an implicit sex classification task. Midline LPP amplitudes were significantly larger for valence rating than for sex classification. During valence rating, faces with a positive valence resulted in larger LPP amplitudes at centrofrontal electrodes than faces with a negative valence. During sex classification, a similar valence effect was observed at midline parietal electrodes. This implicit LPP valence effect appears to depend on higher visual processing, as during an additional sex classification task with blurred faces no such implicit valence effect was found.