First (DI) and second (D2) order derivative spectrophotometric method with an application of base line to peak technique was used for determination of active pharmaceutical ingredients (API) at two wavelengths: fluphenazine (D1 at lambda = 251 nm and lambda = 265 nm, D2 at lambda = 246 nm and lambda = 269 nm), pernazine (D1 at lambda = 246 nm and lambda = 258 nm, D2 at lambda = 254 nm and lambda = 262 nm), haloperidol (DI at = 235 nm and lambda = 253 nm, D2 at lambda = 230 nm and lambda = 246 nm), and promazine (D1 at lambda = 246 nm and lambda = 251 nm, D2 at lambda = 255 nm and lambda = 262 nm). Linear dependence of derivative values on analyte concentration is maintained in a range 3.12 microg x mL(-1) - 44.80 microg x mL(-1). Detection and determination limits are in the range 0.51 - 3.23 microg x mL(-1) and 1.27 microg x mL(-1) - 9.80 microg x mL(-1), respectively. Determination results of drug constituents are very accurate. Recovery percentage is in a range 95.50% - 103.60%.