The effects of the polyamines putrescine, spermidine and spermine on the mechanical activity in smooth muscle from guinea‐pig taenia coli were investigated. Tissue concentrations of spermidine and spermine were about 240 and 160 nmol g^‐1, respectively, while the putrescine concentration was much lower (4 nmol g^‐1). Intracellular [Ca²⁺] was determined using the Fura‐2 method and the effects of extracellularly applied polyamines were observed. Spermine (10^‐5–10^‐3mol l^‐1) dose‐dependently inhibited spontaneous contractions as well as contractions induced by 20–25 mmol l^‐1KCl, with 10^‐3mol l^‐1spermine giving nearly total relaxation. Spermidine (10^‐3mol l^‐1) relaxed a 20 mmol l^‐1KCl contracture, while putrescine had no effect. A decrease in intracellular [Ca²⁺] accompanied the relaxation induced by spermine. In contrast to the effects on spontaneous activity and intermediate [KCl], both the contraction and the accompanying increase in intracellular [Ca²⁺] induced by 90–120 mmol l^‐1KCl were unaffected by spermine. Spermine did not affect the maximal Ca²⁺‐induced force of chemically skinned taenia coli preparations, but a small potentiating effect on the Ca²⁺sensitivity was observed. The results indicate that the relaxation induced by spermine depends on an intact cell membrane. Furthermore, the relaxing effect is due to decreased intracellular [Ca²⁺] and seems to occur only when the contraction is elicited by repetitive action potentials, and not by sustained depolarization.