Nuclei have been prepared from the oviduct of the adult laying hen which are capable of synthesizing large amounts of RNA for long periods of time. The time course of RNA synthesis is linear through 3 h of incubation after an initial burst of activity and is inhibited 60-70% by alpha-amanitin. Maximum synthetic activity requires the presence of serum albumin to stabilize the nuclei, high concentrations of the four ribonucleoside triphosphates, and an incubation temperature of 25 degrees C for continued linear synthesis beyond 30 min. The RNA synthesized in vitro is predominantly 10-20 S with a small proportion of higher molecular weight product. Much of the 10-20S RNA is probably transcribed by RNA polymerase II and is of a size comparable to ovalbumin mRNA. A fraction of this RNA appears to contain poly(A) sequences suggesting that there is some processing of the newly synthesized RNA. These nuclei may provide a useful system for studying the control of the transcription and maturation of ovalbumin mRNA in vitro.