The yawning response to the dopamine (DA) receptor agonist apomorphine HCl (Apo, 7 μg/kg s.c.) and placebo (physiological saline) were examined in two groups of normal men. One group (n = 11) was investigated in the morning and the other group (n = 16) in the afternoon. The frequency of yawning was polygraphically monitored for 60 min following injection. Apo increased yawning compared with placebo when given in the morning (p < 0.02), but not when given in the afternoon. Yawning frequency was increased after both Apo (p < 0.01) and placebo (p < 0.025) when given in the morning compared with responses in the afternoon. These results suggest that yawning frequency with both Apo and placebo is influenced by time of day, possibly as a result of diurnal variation in DA receptor sensitivity.