The effect of metergoline, a 5-hydroxytryptamine (serotonin) antagonist, on the plasma concentrations of prolactin in overtly pseudopregnant Afghan hounds and on the clinical symptoms of overt pseudopregnancy were studied. Plasma concentrations of prolactin and progesterone were determined in six Afghan hounds with signs of overt pseudopregnancy for 2-3 weeks and in three Afghan hounds that were not pseudopregnant at the time of blood sampling. In the overtly pseudopregnant bitches the plasma concentrations of prolactin before treatment (35.5 +/- 8.5 micrograms l-1) were significantly higher than the plasma concentrations of prolactin of the three bitches that were not pseudopregnant (6.3 +/- 0.5 micrograms l-1); the latter values were similar to those of non-pseudopregnant beagle bitches during the total luteal phase. The six pseudopregnant Afghan hounds were treated for 10 days with the antiserotoninergic drug metergoline. At 2 h after the onset of treatment with metergoline, the mean plasma concentration of prolactin had decreased to 10.8 +/- 2.9 micrograms l-1. The plasma concentrations of prolactin continued to decline to 5.4 +/- 1.0 micrograms l-1 at 4 h and to 1.0 +/- 0.1 microgram l-1 during treatment days 3-10. Signs of pseudopregnancy, such as swelling of the mammary glands and digging, decreased during the treatment period. The treatment was associated with mild behavioural side effects such as whimpering and aggressiveness. These side effects are probably not related to suppression of prolactin but are due to a direct effect on serotoninergic pathways in the brain. It is concluded that high plasma concentrations of prolactin are associated with the development and maintenance of pseudopregnancy. The serotonin antagonist metergoline strongly suppresses plasma concentrations of prolactin in pseudopregnant dogs and decreases the clinical signs of pseudopregnancy.