Nest-building, a behavioral model shown to be disrupted by hallucinogens, has never been used to answer questions concerning the psychotomimetic effects of delta9-THC. Several fractions of cannabis and tobacco pyrolysis products were tested consecutively in the same procedure. The following drugs were injected i.p. under a saline-drug-saline schedule: d-amphetamine (6 mg/kg), pentobarbital (25 mg/kg), delta9-THC (10 mg/kg, 5 mg/kg, 2.5 mg/kg), the cannabis fractions designated Is (water soluble products), IIs (nonsoluble, nonvolatile products, IIIs (it comprises what is inhaled by a common hashish smoker), and analogous fractions of tobacco pyrolysis products designated IIIB (what is inhaled by a common tobacco smoker), IIB and IB. The effects of delta9-THC (10 mg/kg), IIs, and IIIs were quite similar as far as the disruption of the normal behavioral pattern is concerned. d-Amphetamine, delta9-THC (5 mg/kg), and IIB disrupted the normal behavioral pattern as well. The similarity of the effects of IIs and IIIs was unexpected in view of the different contents of cannabinoids in these fractions. Also unexpected was the similarity of the effects of delta9-THC (10 mg/kg) and IIIs (40 mg/kg containing 7% delta9-THC) as well as the activity of fraction IIIB.