BACKGROUND Heart period sequences are composed of recurrent, similarly shaped, physiologically meaningful, transient heart period structures. One such structure is a large magnitude transient bradycardia (LMTB), which resembles published reports of responses to alerting stimuli. Alerting responses are sensitive to alpha1-adrenoceptor and cholinergic receptor blockade. OBJECTIVE To determine whether LMTBs are related to alerting responses by assessing the response of rabbit LMTBs to autonomic inhibitors. METHODS Using a Latin square, crossover design, six conscious, unsedated rabbits received placebo, intramuscular glycopyrrolate 0.05 mg/kg and intravenous prazosin 0.15 mg/kg, and six rabbits received placebo, intravenous metoprolol 0.15 mg/kg and intravenous ICI-118,551 0.1 mg/kg. Heart period and blood pressure were continuously recorded, digitized and analyzed off-line. RESULTS LMTBs were suppressed by 93% and 95% by alpha1-adrenergic and muscarinic blockade, respectively, but not by beta1-adrenergic or beta2-adrenergic blockade. The drugs altered none of the morphological parameters of shape, magnitude or duration, or any of the hypotensive features. CONCLUSIONS LMTBs are similar to alerting responses; they are both expressed as transient hypotensive and bradycardic events that are markedly reduced by muscarinic and alpha1-adrenoceptor blockade but not by beta-adrenoceptor blockade. The presence of LMTBs may indicate ongoing, subtle changes in the laboratory environment.