In the rabbit, microinjections of DSIP (delta-sleep peptide) at optimal doses in the median thalamus, periaqueductal gray matter (22.5 nmol), and nucleus of the tractus solitarius (15.0 nmol) produce slow-wave sleep with abundant recruiting spindles. As is seen with morphine, this effect is blocked by naloxone (160 micrograms intracerebrally; i.c.). At least in the rabbit, DSIP is probably a neurotransmitter or a neuromodulator of the bulbothalamic system inducing slow-wave sleep and particularly recruiting spindles. It is likely to restore slow-wave sleep after supraoptimal awakening i.c. microinjections of morphine.