THREE new inter-related histamine alkaloids, together with the previously known alkaloid, Na-cinnamoylhistamine, have been isolated from the leaves of a New Guinea Glochidion species, probably G. philippicum (Cav.) C. B. Rob. (Euphorbiaceae). The new alkaloids are glochidine (I), glochidicine (II), and Na-(4-oxodecanoyl)histamine (III). Glochidine (C₁₆H₂₇N₃O₂, m.p. 65-67°, [α] (CHCl₃) 0°) is a new type alkaloid, shown to be (I); its hydrolysis with concentrated hydrochloric acid at 100° affords histamine hydrochloride and 4-oxodecanoic acid, with IR showing a five-membered-ring lactam band at 1690 cm⁻¹ (CHCl₃) and no NH absorption, and NMR signals at δ 7.42 (C-3 imidazole proton) and 6.72 (C-1 imidazole proton). Glochidicine (C₁₆H₂₇N₃O₂ hemihydrate, m.p. 103-105°, [α]ᴅ (CHCl₃) 0°) is stable to refluxing concentrated hydrochloric acid, shown to be (II); its IR has imidazole NH band at 3500 cm⁻¹ and lactam carbonyl at 1670 cm⁻¹ (CHCl₃), NMR has a deuterium oxide-exchangeable proton signal at δ 9.33 (imidazole NH) and C-2 proton singlet at δ 7.38. Both (I) and (II) have broad eight-proton signals at δ 1.25 (four methylene groups) and broadened three-proton triplet at δ 0.85 (terminal methyl) in NMR, and mass spectra with molecular ion at m/z 261 and major peak at m/z 176 (M - 85, loss of n-hexyl chain at C-4). Na-(4-oxodecanoyl)histamine (III, C₁₆H₂₇N₃O₂, m.p. 117-118°) hydrolyzes with concentrated hydrochloric acid to histamine hydrochloride and 4-oxodecanoic acid, with IR showing amide carbonyl at 1665 cm⁻¹ and ketonic carbonyl at 1710 cm⁻¹ (CHCl₃). When (I) is heated in 10% aqueous acetic acid, a mixture of (I), (II), and (III) is obtained; prolonged boiling affords essentially pure (II). Histamine and 4-oxodecanoyl chloride react in pyridine to give the three bases mixture. (I) and (II) are alternative cyclisation products of (III) via ring closure to imidazole NH or CH respectively. The ratio of the three alkaloids in leaves is uncertain due to acid-induced conversion, but (I) is the major leaf alkaloid. Glochidion bark contains (II) and Na-cinnamoylhistamine, but not (I) or (III).