Nifekalant hydrochloride, a class III antiarrhythmic agent, has recently been placed on the market in Japan. Since its solubility is poor in solutions above pH 6, it is predicted that when nifekalant hydrochloride is dissolved in such solutions at high concentrations, some crystallization may occur. Crystallization of nifekalant and changes in pH were observed when 0.1N-sodium hydroxide solution was added at concentrations of 2, 3, 4, and 5% to a nifekalant hydrochloride solution in either normal saline or 5% glucose. To evaluate the effect of fresh blood on the crystallization of nifekalant, nifekalant or lidocaine (for comparisons with nifekalant), concentrations in the upper layer and clot of blood were measured after fresh blood and nifekalant or lidocaine solution at concentrations of 1, 2.5, 5 and 10mg/mL, respectively. As a result, the more the pH or concentration increased, the greater the degree of crystallization of nifekalant in the solution with a pH between 5.5 and 6.5. The concentration of lidocaine in the upper layer was the same in each solution. Although the concentrations of nifekalant in the upper layer did not differ in the solutions with concentrations of 1 and 2.5mg/mL, remarkable decreases in the concentration of nifekalant in the upper layer were observed in the solutions with concentrations of 5 and 10mg/mL. On the other hand, remarkable increases in the concentration of nifekalant in the clot of blood were observed in the solution with a concentration of 5 and 10mg/mL and crystallizations of nifekalant were observed in specimens of the clots. These results suggested that when a nifekalant injection is administered it should not be mixed with solutions demonstrating a pH level above 5.5 and such concentrations must be maintained at a volume of less than 2mg/mL.