Neuronal networks cultured on microelectrode array (MEA) neurochips provide a testing platform for neuroactive compounds such as neurotoxicants and neuropharmaceuticals. Electrical network activity is recorded, quantified, characterized and classified at the level of spike and burst patterns, yielding concentration–response curves of a multitude of activity-describing parameters for monitoring the effects on defined network activity states. The system allows highly complex studies of cellular behavior which can give new insight into the molecular mechanisms of drug or toxicant action. The MEA neurochip technology is suitable to study network electrical activity maturation (weeks 1–4), the long-term functioning of the active networks (many months), and give a precise characterization of toxicant or drug-induced effects on the electrical activity pattern.