Network behaviour within dorsal horn in vivo is implicated in spinal nociceptive signalling (Sandkühler & Eblen-Zajjur, 1994). In substantia gelatinosa (SG) of spinal dorsal horn in vitro, rhythmicity can manifest in the 4–12 Hz frequency band and is dependent upon chemical and electrical neurotransmission (Asghar et al. 2005). The aim of this study was to investigate modulation of network activity within the SG by drugs that selectively target cannabinoid (CB) receptors. Wistar rats (12–15 days) were terminally anaesthetised with urethane (2 g/kg i.p.). Transverse lumbar spinal slices (300 μm) were cut and field recordings were made from SG. Transient pressure ejection of potassium (KCH₃SO₄, 1.5 M) evoked oscillations with a peak frequency of 7.8 ± 0.1 Hz (mean ± S.E.M.) and a duration of 12.4 ± 2.0 s (n=24). Perfusion of the broad–spectrum cannabinoid agonist CP55940 (1 μM, n=6) significantly attenuated (P < 0.05, paired t test) the power area (by 52 ± 8%) and power amplitude (by 58 ± 8%) of the rhythm. This cannabinoid inhibitory effect was reversed after drug removal. The CB1 specific receptor agonist ACEA (20 nM, n=6) caused a significant decrease (P < 0.05) in the power area (by 44 ± 8%) and amplitude (by 60 ± 8%) of the network activity. AM251 (1 μM, n=4), a CB1 receptor antagonist, had no significant effect on the power area or amplitude when tested alone (P > 0.05). However, AM251 (n=7) abolished the ACEA–induced inhibition of the rhythm (power area by 14 ± 7% and power amplitude by −12 ± 10%, P > 0.05). The results of this study demonstrate an inhibition of dorsal horn rhythmicity upon cannabinoid agonist application. These pharmacological data suggest that this modulation may be attributable to CB1 receptors that are localized to the dorsal horn (Farquhar-Smith et al. 2000). Further studies are required to clarify whether CB1-induced modulation of rhythmic behaviour is linked to the established anti-nociceptive role of cannabinoids in the SG.