Abstract The number of α- and β-adrenergic, muscarinic cholinergic, opiate, and benzodiazepine receptors in rat forebrain, and dopamine and benzodiazepine receptors in striatum, change throughout the day. The diurnal rhythms of these receptors were altered by treatment with the monoamine-oxidase inhibitor clorgyline: following treatment some or all rhythm characteristics of wave form, amplitude, 24-h mean, and phase, were affected. One common effect of treatment was a delay in phase-position of binding to α- and β-adrenergic, opiate and benzodiazepine receptors. Additionally, the nocturnal elevation in pineal melatonin which normally returns to baseline at light onset, persisted 3 h into the light period after clorgyline administratio. These biochemical observations extend bevavioural findings that clorgyline can delay the phase-position of rodent nocturnal activity onset, and does so by slowing the central circadian pacemaker.