For the quantitative determination of endogenic rat homovanillic acid (HVA) concentrations in serum, plasma, tissue homogenates, cerebrospinal Fluid (CSF). This analysis averages the replicate readings for each sample and subtracts the average zero optical density. The standard data points (concentration vs. corrected measurement) are plotted on semi-log axes and a four paramater logistic curve (4PL) is fitted through the points. The concentrations of the samples are determined from the fit with any specified dilution factors applied. The %CV, Standard Deviation and Standard Error are calculated for each replicated sample. Samples outside the range of the standards or the fit (greater than the upper asymptote or below the lower asymptote) are highlighted in yellow.