The search for drugs that modulate NMDA receptors has been very active in the latest decades since they play a pivotal role in neuronal death and in several neurological conditions, including stroke, traumatic brain injury, and neurodegenerative diseases. Activating the receptor evokes an entry of calcium into the neuron, thus leading to a rapid depolarization that can be measured using electrophysiological methods or by calcium imaging. Although several attempts have been made to develop high-throughput screening assays based on electrophysiological patch-clamp or imaging methods, these solutions amply rely on costly or proprietary equipment and are not suitable for independent or small laboratories that want to test a few dozens of compounds acting on NMDA receptors. Here, we describe an easy and cost-effective method that delivers reliable results using equipment that is normally accessible to any biology or pharmacology laboratory and that can be adapted to different instruments, like fluorescence microscopes or fluorescence plate-readers. Cell cultures are the easiest way to provide a biological substrate that behaves at least similarly to neurons in a living organism. Here, we show how to obtain primary cultures of cerebellar granule cells that are a suitable biological substrate to measure the activity of compounds on NMDA receptors using the calcium-sensitive indicator Fura-2.