p,p'-DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane; 75 mg/kg) or corn oil was administered po to male Fischer 344N rats. Tremor was quantified 8 hr later by spectral analysis of whole body movements. The effect of sc injection of pharmacological challenges on the spectral profile of body movements was determined. The alpha antagonist phenoxybenzamine decreased the intensity of movements over most of the power spectra in animals exposed to DDT, but decreased spectral power only at lower frequencies in control subjects. The alpha-1 antagonist prazosin had similar effects in animals given DDT. The alpha-2 antagonist yohimbine and the beta antagonist propranolol produced lethality and increased the intensity of movements in animals administered DDT, without significantly affecting control animals. The alpha-2 agonist clonidine decreased the spectral profile over a wide range of frequencies in animals exposed to DDT, while depressing the spectral power of control animals only at higher frequencies. The dopamine antagonist haloperidol increased the intensity of movements in DDT-treated animals, without altering the spectral profile of controls. The dopamine agonist apomorphine induced stereotypy in control animals, but failed to significantly alter the power spectra in subjects given DDT. These data suggest a facilitatory and inhibitory role, respectively, for alpha-1 and alpha-2 receptors in the modulation of DDT-induced tremor. Dopamine and beta receptors may be involved in the tonic inhibition of tremor produced by DDT.