The use of wide-bore capillary columns in gas chromatography (GC) with nitrogen-phosphorus detectors (NPD) is gaining popularity in the toxicology laboratory. Though the preferred method to achieve reproducible results and to make interlaboratory comparisons of GC data is by retention index (RI), the selectivity of the NPD has relegated its users to calculations of relative retention time. The present study utilizes a set of drugs as reference standards under temperature programmed conditions and presents a unique method of RI calculation. RI calculations are highly reproducible with this technique (day-to-day variations range from 0.3 to 3.4 RI units) and are comparable to packed column, FID generated reference data. A program, written in Basic, calculates RI values based on daily injections of the reference standards and searches a library of over 100 basic and acidic drugs.