Abstract The effects of amphetamine, LSD and delta-9-tetrahydro-cannabinol (THC) on the ant were studied. Methods Starved worker ants of the species Formica pratensia were fed with dl-amphetamine (A, 0.1-50 mg/ml), dl-methamphetamine, LSD (Sandoz, 1 mcg-l mg/ml) or THC (1 mg/ml) or were narcotized with ether and A (0.005, 0.5, 5 or 50 mg/ml) or LSD (1, 10 or 100 mcg/ml) was injected into the abdominal cavity. Behavior was observed 2 hr after drug administration. The preference of ants for control food or food drugged with LSD (100 mcg/ml) or THC (1 mg/ml) was tested. The distribution and group formation of ants was studied 2, 9, 24 and 48 hr after feeding with LSD (100 mcg/ ml) or THC (1 mg/ml) for 1 hr. Results Injected A (1-10 mcg/ant) increased aggression and caused writhing and convulsions. At 100 mcg/ml, A caused fatal paralysis. Food containing A or methamphetamine was accepted only at low concentrations with no behavioral effects. Locomotion became uncoordinated after receiving LSD, by either route at a dose of 20 ng/ant. Higher doses caused death. THC did not cause any significant behavioral changes. There was a slight preference for sucrose alone over a solution containing LSD, but THC did not affect the acceptability of sucrose solutions. LSD reduced groupformation slightly, but THC had no effect.