Abstract: Microtubule disrupter, colchicine, and microtubule stabilizer, taxol, were used to determine whether microtubules play a role in β-adrenergic receptor mRNA homeostasis and agonist-induced down-regulation in C6 glioma cells. Colchicine treatment had significant, differential, time-dependent effects on constitutive β1- and β2-adrenergic receptor mRNA levels. These effects stemmed from the action of colchicine on microtubules, because β-lumicolchicine, an inactive isomer, had no effect, and nocodazole, a structurally unrelated microtubule disrupter, had similar effects. Colchicine treatment had little effect on the total number of β-adrenergic receptor binding sites as measured by (−)-[125I]iodopindolol binding, but did alter the relative proportion of β1- and β2-adrenergic receptor subtypes. Colchicine also had no effect on basal cyclic AMP levels. In contrast to colchicine, taxol treatment had little long-term effect on either β1- or β2-adrenergic receptor mRNA levels. Taxol antagonized the effects of colchicine on total binding and mRNA levels. Taxol treatment increased basal cyclic AMP levels fourfold and potentiated (−)-isoproterenol-induced cyclic AMP production. Colchicine pretreatment completely inhibited (−)-isoproterenol-induced down-regulation of β1-adrenergic receptor mRNA, but not that of β2-adrenergic receptor mRNA. Taxol pretreatment had little effect on isoproterenol-induced β-adrenergic receptor mRNA down-regulation. Colchicine pretreatment also attenuated isoproterenol-induced receptor down-regulation and inhibited agonist-stimulated cyclic AMP production. These effects of colchicine were antagonized by taxol. Whereas the effects of taxol and colchicine on isoproterenol-induced down-regulation of β-adrenergic receptor mRNA are consistent with their effects on cyclic AMP production, those of colchicine in the absence of stimulation must involve other mechanisms. The data demonstrate that the state of microtubule assembly can affect cyclic AMP levels, β1- and β2-adrenergic receptor mRNA, and binding site levels in C6 glioma cells.