How pathological tau — a feature of several neurodegenerative diseases — affects neuronal network function is not clear. The authors took electrophysiological recordings of the activity of frontal cortex pyramidal neurons in mice with forebrain expression of human mutant tau, before the animals exhibited neuronal cell death. Tau transgenic neurons exhibited slower subthreshold membrane oscillations during slow-wave sleep, longer hyperpolarized 'down' states and lower firing rates during 'up' states than did control neurons. These data imply that tau impairs membrane dynamics and activity in intact networks.