Chronic THC occludes HFS-LTD. We examined the effects of administering THC via intraperitoneal injections. A, A single THC injection failed to occlude LTD (n = 9 from 7 mice; p < 0.05 compared with baseline at 10–15 and 15–20 min). B, Chronic injections (7–10 consecutive days) of THC occluded LTD following HFS (n = 6 from 5 mice, p < 0.05, compared with vehicle only injections). C, Chronic injections of EtOH vehicle as a control demonstrate continued LTD (n = 6 from 5 mice, p < 0.05, compared with chronic THC).