Timely deactivation of the α-subunit of the rod G-protein transducin (Gαt) is essential for the temporal resolution of rod vision. Regulators of G-protein signalling (RGS) proteins accelerate hydrolysis of GTP by the α-subunits of heterotrimeric G proteins^,,in vitro. Several retinal RGS proteins can act in vitro as GTPase accelerating proteins (GAP) for Gαt^,,,. Recent reconstitution experiments indicate that one of these, RGS9-1, may account for much of the Gαt GAP activity in rod outer segments (ROS)^,. Here we report that ROS membranes from mice lacking RGS9-1 hydrolyse GTP more slowly than ROS membranes from control mice. The Gβ5-L protein that forms a complex with RGS9-1 (ref. ) was absent from RGS9^-/- retinas, although Gβ5-L messenger RNA was still present. The flash responses of RGS9^-/- rods rose normally, but recovered much more slowly than normal. We conclude that RGS9-1, probably in a complex with Gβ5-L, is essential for acceleration of hydrolysis of GTP by Gαt and for normal recovery of the photoresponse.