Morphine (1 microgram) was microinjected into rats in the midline medullary nucleus raphe magnus (NRM); 1 mm lateral into nucleus reticularis paragigantocellularis (NRPG); 2 mm lateral into the VIIth nerve nucleus and 3 mm lateral into the Vth nerve nucleus. The time course of changes in the sensitivity to noxious heat was followed by the tail flick test. Significant and prolonged antinociception was seen following microinjection into NRPG. At sites 1 mm from NRPG very weak effects were seen and at 2 mm from NRPG no antinociception occurred. It is concluded that 1 microgram of morphine microinjected into the brainstem is unlikely to cause antinociception by entering the circulation and having effects at remote sites. The distance diffused by morphine to cause significant antinociception after microinjection of 1 microgram is less than 1 mm. Levorphanol (1 microgram) had very similar effects to morphine but dextrorphan and saline were ineffective. It is concluded that although the concentrations achieved following microinjections may be high, they are not excessive as the effects show stereospecificity. The concentrations of endogenous substances released into the synaptic cleft may also be high.