Twenty-four male mice, belonging to 8 different inbred strains, were tested in an 8-arm radial maze. Clear strain differences were found for performance on the third day of training, which correlated very strongly with the size of the hippocampal intra- and infrapy ramidal mossy fibre (iip-MF) terminal fields. These results, combined with those from earlier experiments, indicate that genetic variations of the iip-MF projection influence processes that determine behavioural abilities of mice.