The response of lambda-phage DNA molecules to a well-defined elongational flow field generated by a Taylor four-roller mill was investigated by observing the flow birefringence, Delta n. Delta n in the center of the four rollers, near the stagnation point, was localized at the mill exit symmetry plane. The intensity gradually increased from the off-symmetrical plane to the center of the mill, and at the exit symmetrical plane, the intensity was maximum. Delta n also gradually increases with the strain rate, epsilon. These observations indicate that DNA molecules in the solution would be free draining in nature. From the decay of Delta n at each point in the mill after a sudden stop of the mill operation at 24 s(-1), the rotational diffusion coefficient of molecules, D-r, at each point in the mill space was estimated, where the relaxation time of the decay of Delta n was considered to be related to the molecular disorienting process. It is concluded that at 24 s(-1) lambda-phage DNA molecular coils near the stagnation point, which was assumed to be a prolate spheroid as a whole, was so deformed that the aspect ratio p (=b/a less than or equal to 1, where a and b are, respectively, the longer and shorter axes) would be 1/12 of that of the DNA molecule which has just entered the mill space. This result suggests that there is a possibility for the DNA molecule to be in a stretched conformation at a higher strain rate.