Locomotor activity of young adult (8 ± 1 months) and aged (18 ± 2 months) GFRα-1+/− and wild-type mice over a 30-min interval following injections of saline or the D1 receptor agonist SKF 82958 (SKF). Following saline control injections, GFRα-1+/− mice were less active than wild-type controls at both ages, and activity declined with age. SKF elevated motor activity at both ages, and to a greater extent in the older than in the young adult animals. Importantly, despite the lower basal activity levels for the older GFRα-1+/− mice following saline injections, their response to the D1 agonist was greater than that of comparable aged wild-type mice at the highest SKF dose. Follow-up anova results are indicated (*P < 0.05). (See Results for further information on statistical analysis.)