This paper summarizes a study on the time-dependent growth of complex viscosity during the anionic ring-opening polymerization of caprolactam employing isophthaloyl-bis-caprolactam and caprolactam-magnesium-bromide as initiator and catalyst. The complex viscosity of the system-nylon 6 polymerizing in its monomer-has been defined over the temperature range of 120-160 degrees C by in situ mixing and polymerizing two caprolactam streams, one containing initiator, the other containing catalyst, between oscillating platens of a Rheometrics rheometer. Isothermal rheometry at five temperatures was accomplished for one initiator/catalyst combination.