Color discrimination ability of 100 in-patients suffering from congestive heart failure and treated with digitoxin (D), pengitoxin (P), or digoxin (Dg) was determined with the Farnsworth-Munsell 100 Hue test (FM 100) and compared with the color discrimination of 72 in-patients who were not treated with digitalis glycosides (control group C). Parallel to the performance of the FM 100, the glycoside plasma level was measured by radioimmunoassay. The total error score (TES) of the FM 100 was correlated with the glycoside plasma level and the patient's age. In the C as well as in the D or P groups up to 172 errors and in the Dg group up to 586 errors were observed. With the exception of Dg, no differences were observed between the regression lines indicating an age-dependent increase in TES even under D or P treatment. In contrast to the two glycosides, Dg enhances the TES in therapeutically relevant plasma concentrations. The differences between the glycosides are due to differences in their volume of distribution and their plasma protein binding.