The antipanic utility of imipramine and monoamine oxidase inhibitors has led to hypotheses of noradrenergic and/or serotonin (5-HT)-related abnormalities underlying panic disorder (PD) and its agoraphobic complications. Further data support significant antipanic effects for agents with acute 5-HT reuptake blockade effects. It is unlikely that ameliorative effects observed following chronic administration of 5-HT drugs remain 5-HT specific. Despite a range of recently discovered 5-HT receptor subtypes, evidence for a specific receptor abnormality in PD is lacking. Preclinical studies suggest an important role for 5-HT effects in several animal models of anxiety, including separation-induced infant protest responses and respiratory modulation. Induction of anxiety with putative 5-HT agents such as meth-chloro-phenylpiperazine and fenfluramine lend further support to 5-HT involvement in anxiety states. Critical behavioral, physiologic, and neuroendocrine differences between putative 5-HT anxiogens and "classic" panicogens, such as lactate and carbon dioxide are discussed. We propose that 5-HT agents mediate antipanic effects through amelioration of a deranged internal evaluative mechanism along cybernetic lines, rather than simple augmentation or reduction of 5-HT function.