Now that the pharmacology and neuronal connectivity underlying REM sleep is beginning to be understood, it is important to begin investigations that elucidate the transcriptional response related to the REM sleep process. The present study focuses on determining the temporal development of Fos-like immunoreactivity (Fos-LI) in the dorsolateral pons following cholinergically induced, sustained rapid eye movement (REMc) sleep in cats. Microinjections (0.25 microliter) of vehicle (n = 3) or carbachol (0.2–4.0 micrograms/0.25 microliters) were made into the medial pontine reticular formation. Carbachol produced a state with all the signs of natural REM sleep, and with durations from 0 min to 120 min. Animals were killed either immediately or at various intervals after the end of REMc. Compared to vehicle- and carbachol- treated animals without REMc, the animals with REMc showed a significantly higher number of Fos-LI cells in pontine regions that have been implicated in REM sleep generation. Regions with REMc- associated Fos-LI increases included the lateral dorsal tegmental (LDT) and pedunculopontine tegmental (PPT) nuclei; the locus coeruleus; the dorsal raphe; and the medial pontine reticular formation. More Fos-LI cells were found with longer REMc bouts than with shorter-duration REMc bouts. However, with 2 hr long REMc bouts the number of Fos-LI cells returned to control levels, suggesting that the c-fos transcriptional cascade is turned off once a threshold of REMc has been reached. These findings indicate that pontine neuronal populations implicated in REM sleep express more c-fos in the course of REMc, and that the extent of expression is related to the duration of the state.