It has long been known that the intracerebroventricular administration of ACTH1-24 increases the duration of grooming episodes and the frequency of yawning in rats. The objective of this study was to investigate in what way these episodes are prolonged and whether and to what extent genotype influences such effects. We compared the effect of increasing doses of intracerebroventricular injections of ACTH1-24 on grooming and yawning in males of two inbred strains of Sprague-Dawley rats with distinct yawning frequency, high-yawning (HY) and low-yawning (LY). In LY rats the duration of grooming episodes increased, while in HY rats grooming episodes augmented both in number and duration. In LY rats the duration of grooming components increased likewise, in HY rats however, neither the number nor the duration of the components changed. The grooming rate in both strains of rats was slowed, though more so in LY than in HY rats. Yawning increased in LY rats but not in HY rats. We conclude therefore that ACTH1-24 increases the duration of grooming episodes by slowing the grooming rate according to genotype, and may or may not alter the frequency of yawning.