MF-268 bitartrate [(3a S, 8a R)-1,2,3,3a,8,8a-hexahydro-1,3a,8-trimethylpyrrolo[2,3-b]indol- 5-ol[8-(cis2,-6-dimethyl-morpholin-4-yl)octyl]-carbamate L-bitartrate hydrate; Mediolanum Farmaceutici, Milan, Italy] is a pseudo-reversible carbamate-type cholinesterase inhibitor (ChEI) which interacts with the catalytic and regulatory anionic site of the enzyme. Its effects on extracellular levels of acetylcholine (ACh), norepinephrine (NE), dopamine (DA), and serotonin (5-HT, 5-hydroxytryptamine) were studied in rat cortex by using a microdialysis technique coupled with high-performance liquid chromatography-electrochemical detection (HPLC-ECD). Conscious, freely moving rats were systemically [per os (p.o.) and subcutaneously (s.c.)] administered MF-268 with no ChEI in the probe. Cholinesterase inhibition in brain was assayed in parallel experiments. Oral administration of MF-268 (0.5, 2.0, and 5.0 mg/kg) produced a significant increase of extracellular ACh in cortex; the maximal increase was 300% [not significant (n.s.)], 460% and 1,200%, respectively. Maximal cholinesterase (ChE) inhibition was 2.3% (n.s.) at 9 hr and 9.7% (P < .05) at 12 hr after the 2.0 and 5.0 mg/kg doses, respectively. Norepinephrine and DA levels were increased 180% and 100% after the 5.0 mg/kg dose, respectively; 100% and 60% after the 2.0 mg/kg dose, respectively; and 70% for both amines after the 0.5 mg/kg dose, respectively. The elevation lasted at least 5 hr with the 2.0 and 5.0 mg/kg doses. There were no major changes in 5-HT levels at these three doses. Subcutaneous administration (0.5 and 2.0 mg/kg) produced a maximal 360% (5.5 hr) and 2,500% (5 hr) increase in extracellular ACh, respectively. Maximal ChE inhibition was 13% (0.5 mg/kg) and 41% (2.0 mg/kg). Neither 0.5 nor 2.0 mg/kg produced a consistent modification of NE. Only a transient increase in DA was seen with the 0.5 mg/kg dose. There were no changes in 5-HT levels at these two doses. MF-268-treated animals showed slight cholinergic side effects (chewing, tremor) at both doses. MF-268 administered intracortically through the microdialysis probe at a concentration of 50 microM induced a 5,900% increase in ACh levels at 6 hr. This effect started 30 min after injection and continued throughout the period of administration. MF-268 produced a significant decrease in NE levels (-44%) starting at 30 min, and a slight but significant increase in DA levels of 45% at 2.5 hr. A significant increase of 5-HT (58%) was also observed starting at 4 hr. Slight symptoms of cholinergic toxicity were observed during intracortical administration.