It has been repeatedly shown in cats that acute administration of carbachol into the pontine reticular formation (PRF) readily evokes a state that closely mimics natural paradoxical sleep (PS). Surprisingly, there are few corresponding studies in rats. In order to further characterize the effects of pontine carbachol in rats, 151 injections of different doses (from 3 micrograms to 0.005 microgram in 0.1 microliter saline) of carbachol were made at different sites within the PRF of 70 rats. Sleep-waking states obtained in the 4 hours following carbachol administration were compared to control values, obtained both under baseline condition (no injection) and following pontine injection of 0.1 microliter saline. On the one hand, from the whole set of carbachol injections, it appeared that: 1) most injections (112/151) did not significantly alter the sleep-wake states; 2) when carbachol was effective, it induced either increased PS (20 injections) or increased waking (19 injections); and 3) effective injection sites were intermingled with noneffective sites. Dose- or site-dependency effects can account in part, but not totally, for these discordant results. On the other hand, in accordance with previous rat studies, we found that: 1) the PRF medial and ventral to the motor trigeminal nucleus was the most effective region for carbachol to increase PS; 2) carbachol-induced PS enhancement was of moderate magnitude (+60% above control saline level over the 4-hour recording time); 3) latency to onset of the first PS episode was not shortened; and 4) only the number of PS episodes was increased, their duration was not prolonged. These characteristics of carbachol-induced PS enhancement strongly differ, both in terms of magnitude and timing, from those described in cats. We suggest that the less reliable and weaker effects of pontine carbachol injection in rats compared to cats can be due to methodological problems inherent in the intracerebral microinjection technique and also to species-related differences in the mechanisms controlling the PS state.