1 SH-SY5Y cells express alpha7 and alpha3* subtypes of nicotinic acetylcholine receptors (AChR). Numbers of these receptors are upregulated by chronic treatment with nicotinic agonists or KCl. In this study we have examined the functional consequences of these drug treatments on nicotine- or KCl-evoked increases in [Ca2+](i), in SH-SY5Y cells. 2 In untreated cells, nicotine increased [Ca2+](i) (EC50 7.5 mum). Responses to 10 mum nicotine were abolished by the non-selective nicotinic antagonist mecamylamine and were partially blocked by alpha7 selective antagonists, the alpha3beta2*-selective antagonist alpha-conotoxin-MII, and by cadmium and verapamil. 3 After treatment for 4 days with nicotinic agonists, nicotine-evoked increases in [Ca2+](i) were significantly decreased by about 25%. Nicotine-evoked responses were paradoxically increased in the presence of acute methyllycaconitine (MLA; an alpha7-selective antagonist) although other alpha7-selective antagonists were without effect, while alpha-conotoxin-MII gave a partial inhibition. The increase observed with MLA was abolished by mecamylarnine but not by alpha-conotoxin-MII and was still observed 24 h after chronic nicotine treatment. 4 After treatment for 4 days with KCl, nicotine-evoked increases in [Ca2+](i) were also decreased by 25%, but acute MLA was without effect. Responses to 20 mM KCl were unchanged by prior treatment with nicotine or KCl. Treatment for 4 days with 5 mum verapamil reduced responses to both nicotine and KCI by about 50%. 5 Multiple nicotinic AChR subtypes contribute to nicotine-evoked increases in [Ca2+](i) in SH-SY5Y cells. Responses to acute nicotine are reduced after chronic nicotine or KCI treatment, with loss of the component attributed to the alpha7 subtype. However, in nicotine-treated cells this effect is reversed when nicotine stimulation is applied in the presence of acute MLA. The antagonist may assist in converting a non-functional alpha7 nicotinic AChR to a conducting state.