Geodia baretti (Bowerbank) Lamarck is predominant on the deep bottoms in the Koster fjord on the northern Swedish west coast. Chemical investigations of the genus Geodia are very limited. Aqueous and MeOH extracts of G. baretti exhibited strong contractile activity in the isolated guinea-pig ileum assay, while the aqueous extract lacked lectinlike activity. EtOH extracts of freshly collected material were chromatographed on LH-20 to give seven fractions, all showing either contractile activity or inhibition of electrically stimulated guinea-pig ileum. Further investigation of fractions 5 and 6 led to the identification of barettin, adenosine (fraction 6), histamine (responsible for the main activity of the crude extract), inosine (fraction 5), and three N-methylated nucleosides: 3-methylcytidine (3mCyd), 3-methyl-2'-deoxycytidine (3mdCyd), and 3-methyl-2'-deoxyuridine (3mdUrd) (fraction 5), identified by comparison with authentic samples and via fabms, ¹H-nmr, and ¹³C-nmr data. Biological activities showed that only 3mCyd and 3mdCyd exhibited strong contractile activity, while most other nucleosides had no effect or inhibitory effect. These products are present in minute amounts, and their origin (sponge or associated microorganisms) is unclear. 3mdCyd and 3mdUrd are reported as free natural products for the first time.