1. We have extended previous investigations of four analogues of Delta8-tetrahydrocannabinol (Delta8-THC): 6'-azidohex-2'-yne-Delta8-THC (O-1184), 6'-azidohex-cis-2'-ene-Delta8-THC (O-1238) and octyl-2'-yne-Delta8-THC (O-584) and its 1-deoxy-analogue (O-1315). 2. O-1184, O-1238 and O-584 displaced [3H]-CP55940 from specific binding sites on Chinese hamster ovary (CHO) cell membranes expressing CB1 or CB2 cannabinoid receptors, with pKi values of 8.28 to 8.45 (CB1) and 8.03 to 8.13 (CB2). The pKi values of O-1315 were significantly less, 7.63 (CB1) and 7.01 (CB2). 3. All the analogues inhibited forskolin-stimulated cyclic AMP production by CB1-transfected CHO cells (pEC50=9.16 to 9.72). Only O-1238 behaved as a full agonist in this cell line. 4. In mouse vasa deferentia, O-1238 inhibited electrically-evoked contractions (pEC50=10.18 and Emax=70.5%). Corresponding values for O-1184 were 9.08 and 21.1% respectively. At 1 nM, O-1184 produced surmountable antagonism of the cannabinoid receptor agonist, CP55940. However, at 0.1 nM, O-1184 did not attenuate CP55940-induced inhibition of cyclic AMP production by CB1-transfected CHO cells. 5. In CB2-transfected CHO cells, cyclic AMP production was inhibited by CP55940 (pEC50=8.59), enhanced by O-1184 and O-584 (pEC50=8.20 and 6.86 respectively) and not significantly affected by O-1238 or O-1315. 6. At 100 nM, O-1184 and O-1238 produced surmountable antagonism of CP55940 in CB2 cells, decreasing the pEC50 of CP55940 from 8.61 to 7.42 (O-1184) or from 8. 54 to 7.44 (O-1238). 7. These data support the hypothesis that increasing the degree of unsaturation of the aliphatic side-chain of Delta8-THC analogues has little effect on CB1 or CB2 receptor affinity but can reduce CB1 receptor efficacy and reverse the direction of responses elicited at CB2 receptors.