In mice performing an operant task, increases in neural activity in direct- and indirect-pathway spiny projection neurons (SPNs) are associated with action initiation but not with inactivity, and concurrent activation of SPNs from both pathways in one hemisphere precedes the initiation of contraversive movements. The prevailing view of the functional organization of the basal ganglia, brain areas involved in movement control, is that there are two distinct, and opposing, neural circuits that facilitate and inhibit movement. Costa and colleagues have developed a new in vivo technique for monitoring the activity of specific cell types deep in the brain via fibre optics, and use it to monitor activity in these two pathways in freely behaving rodents. Rather than opposing activity, both pathways are active before movements are initiated. This challenges the classical view of basal ganglia function, and may have implications for understanding the origin of motor symptoms in disorders such as Parkinson's disease. The basal ganglia are subcortical nuclei that control voluntary actions, and they are affected by a number of debilitating neurological disorders1,2,3,4. The prevailing model of basal ganglia function proposes that two orthogonal projection circuits originating from distinct populations of spiny projection neurons (SPNs) in the striatum5,6—the so-called direct and indirect pathways—have opposing effects on movement: activity of direct-pathway SPNs is thought to facilitate movement, whereas activity of indirect-pathway SPNs is presumed to inhibit movement1,2. This model has been difficult to test owing to the lack of methods to selectively measure the activity of direct- and indirect-pathway SPNs in freely moving animals. Here we develop a novel in vivo method to specifically measure direct- and indirect-pathway SPN activity, using Cre-dependent viral expression of the genetically encoded calcium indicator (GECI) GCaMP3 in the dorsal striatum of D1-Cre (direct-pathway-specific6,7) and A2A-Cre (indirect-pathway-specific8,9) mice10. Using fibre optics and time-correlated single-photon counting (TCSPC) in mice performing an operant task, we observed transient increases in neural activity in both direct- and indirect-pathway SPNs when animals initiated actions, but not when they were inactive. Concurrent activation of SPNs from both pathways in one hemisphere preceded the initiation of contraversive movements and predicted the occurrence of specific movements within 500 ms. These observations challenge the classical view of basal ganglia function and may have implications for understanding the origin of motor symptoms in basal ganglia disorders.