Abstract The behavioural and EEG effects of harmine, harmaline and 5 related β-carbolines were studied in rats and rabbits bearing chronically implanted electrodes in various cortical and subcortical areas. Based on the results obtained, the 7 derivatives can be divided into 3 groups. The first group includes harmine and harmaline, which caused excitation, tremors, and ataxia both in the rat and in the rabbit. The modifications of the cerebral electrical activity during the tremors consisted of an increase in frequency and in voltage, observed in the cortical leads. Harmol, tetrahydro-harmane and 3-methyl-harmane had a different toxic effect, consisting of a depressive syndrome, which can extend to complete paralysis in the rat. Harmane and nor-harmane produced, at low doses, a motor depression having a catatonic component; higher doses led to clonic and tonic-clonic type convulsions; tremors were never observed. In the rabbit, but not in the rat, l-DOPA was effective in antagonizing the tremors and the other toxic symptoms caused by the two drugs. These results suggest that the tremors induced by harmine and harmaline may be caused by an effect on the extrapyramidal dopaminergic system.