A method for the production of polyacrylamide gel filled capillaries was studied in detail. A polymerizing solution of acrylamide was injected into the capillary without its inner surface pretreatment and polymerized, in situ, by radical initiators. Bubble formation in capillaries was avoided by using well-designed injection equipment, which was developed for use in this study. Performance of gel-filled capillaries was examined in terms of stability, reproducibility of migration time, feasibility of method, and resolving power of polynucleotides. Gel-filled capillaries prepared by this method showed high precision in relative migration time, ultrahigh resolution of polynucleotides, and wide applicability. Average relative standard deviations in migration times for polynucleotides in the chain length range from 50 to 250mer were 1.1% (run to run), 1.5% (day to day), and 2.1% (batch to batch), respectively. Stability of the gel-filled capillary was less than that of a gel-filled capillary in which the gel was chemically bound to the capillary inner surface. A plate number for a gel-filled capillary of 1.5 X 10(7) m-1 was achieved. Mixtures of 450 kinds of polyadenylic acids were baseline-resolved and analyzed within 100 min. High-resolution separation of a mixture of polydeoxyadenylic acids was also achieved. The method was demonstrated to be applicable to the production of gel-filled capillaries with wide varieties of gel composition and capillary diameters. Advantages and limitations of this method are discussed.