We have previously reported that nicotine stimulates cell proliferation of three small-cell lung carcinoma (SCLC) cell lines by activating nicotinic receptors of the neuronal type. Here we report that, in the GLC-8 SCLC cell line, nicotine stimulates mitogen-activated protein (MAP) kinase activity in a concentration- and time-dependent manner (ED₅₀ = 10 nM). The nicotine effect was antagonized by mecamylamine, an antagonist specific for neuronal nicotinic receptors. The absence of extracellular Ca²⁺, or pretreatment with pertussis toxin or the tyrosine kinase inhibitor genistein inhibited the action of nicotine on MAP kinase. Moreover, supernatants from nicotine-stimulated cells transferred to cells pretreated with mecamylamine were still capable of activating MAP kinase. On the other hand, the same supernatants transferred to cells pretreated with mecamylamine and pertussis toxin or genistein failed to activate MAP kinase. These findings suggest that nicotine elicits its stimulatory effect on MAP kinase in SCLC cells indirectly by inducing the production and/or release of a factor which then acts via a pertussis toxin- and tyrosine kinase-sensitive route.