The effect of cannabidiol (CBD), cannabigerol (CBG), cannabinol (CBN), Δ8-tetrahydrocannabinol (Δ8-THC), and Δ9-tetrahydrocannabinol (Δ9-THC) on the in vivo metabolism of [14C]pentobarbital (14C-P) was investigated in rats. The cannabinoids were administered ip (20 mg/kg) 30 min prior to either oral or iv treatment with 14C-P. When 14C-P was given po, the 14C blood concentrations were initially depressed and later elevated by CBG and CBD, increased by Δ8-THC and Δ9-THC and unaffected by CBN. When 14C-P was injected iv, the blood 14C values were elevated by CBD and unchanged by CBG, Δ8-THC or Δ9-THC. Urinary excretion of total 14C and the metabolites of P was decreased by CBD, CBG and Δ8-THC during the first 6 hr following treatment. The effect of CBD on the blood concentration and urinary excretion of 14C was dose-related. In rats treated with CBD + P, the liver and serum concentrations of P metabolites were significantly lower and the liver, serum and brain concentrations of unmetabolized P were significantly higher than in P-treated rats. Pentobarbital induction and sleeping times were potentiated by CBD and Δ9-THC and antagonized by CBG. It was concluded that CBD delayed P metabolism, CBG decreased the rate of P absorption and excretion. Δ8-THC and Δ9-THC decreased elimination of P and CBN had little effect on P absorption, metabolism or excretion. The effect of CBD + P on sleeping time was correlated with brain P concentrations.