Pimavanserin is an antipsychotic with great potential in the treatment of delusions and hallucinations in Parkinson's disease, a very common chronic neurodegenerative disease. The aim of these thesis was examination of selectivity of votammetric method during simultaneous determination of pimavanserin and levodopa in combination with different drugs and substances. For examination was chosen square wave voltammetry technique, and the measurements were made on the surface of glassy carbon electrode, which was modified with nanomaterials (mixture of graphene nanoplates and graphitized multi-walled carbon nanotubes 1:1, concentration of 1 mg/mL, in 0,3% Nafion), with previous preconcentration of analyte for 60 seconds, at the frequency of 75 Hz, with pulse amplitude of 20 mV and increase of potential of 4mV. Determination have proven that pimavanserin and levodopa don't interfere with each other when they are measured with new voltammetric method. Pimavanserin shown current peak at potential of 1,07 V, and levodopa shown current peak at 0,37 V. Linearity ranges for both drugs were also determined. Substances that can be found in body liquids, such as glucose, sodium chloride, potassium chloride, carbonates, fosfates and uric acid, do not interfere in simultaneously determination with pimavanserin and levodopa. Some medications such as quetiapine and ropinirole interfere in determination of pimavanserin, because their oxidation peaks are too close to oxidation peak pimavanserin and with it they interfere. Oxidation peak of dopamine observed at 0,37 V din't affect on possibillity of determination of pimavanserin, but it interfere with possibillity of determination of levodopa at concentration higher than 5x10-7 mol/dm3 , what is significantly higher than expected serum concentration. In the measurements, entacapone did not affect the ability to determine pimavanserin and levodopa, since its current peak at a potential of 0.54 V doesn't interfere with the current peak of pimavanserina or the current peak of levodopa.