The oxidation of sparteine was studied in a total of 121 Ngawbé Guaymí volunteers in Panama, 97 of whom were unrelated. When presented in a frequency histogram, the results of the log10 of the metabolic ratios (LMR) indicated the existence of two modes, the largest of which exhibited a normal distribution (alpha = 0.05; chi 2 = 5.46). A preliminary assignment of an antimode for this population sample is proposed, located within the region of LMR 0.65 to 0.85 vs LMR of 1.3 for white subjects, and results in five poor metabolizers (PMs) (5.2%). This is in contrast to the absence of PMs (0/210) we have reported for the Cuna Amerindians. The microevolution of the sparteine route, corresponding to a tenfold change in the frequency of PMs, is likely to have occurred within their genetic divergence time. These observations of the divergence of a metabolic route of therapeutic importance and the proposal of a time frame for its microevolution constitute the first cases in the literature.