The synthesis and serotonin receptor subtype affinity of a series of ergolines are described. High selectivity for the 5-HT1 subtype was found with a number of 8-substituted (3 beta, 5 beta)-9,10-didehydro-6-methylergolines. The more potent and selective of these compounds increased the concentration of serotonin and decreased the concentration of 5-HIAA in rat brain and increased corticosterone concentration in rat serum. Oral administration of 13, (3 beta)-2,3-dihydrolysergine, produced long-lasting decreases in serotonin turnover. Compound 13 lacked substantial dopaminergic activity as measured by its effects on dopamine turnover in whole brain or striatum and its affinity for alpha-adrenergic binding sites was significantly less than for 5-HT1 binding sites. The increases in serum corticosterone concentrations produced by 13 were not blocked by the serotonin uptake inhibitor fluoxetine or by the serotonin synthesis inhibitor p-chlorophenylalanine, suggesting that 13 exerts its effects through direct stimulation of serotonin receptors.