Paxilline, an indole alkaloid mycotoxin from Penicillium paxilli, is an inhibitor of the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA). Paxilline inhibited differing isoforms of SERCA with IC50s between 5 and 50 microM. It inhibited more potently the purified Ca2+ ATPase activity from skeletal muscle with an IC50 of 5 microM. Detailed effects of this inhibitor on the Ca2+ and ATP dependence upon activity indicate that it affects the high-affinity Ca2+-binding (E1) form of the ATPase. In addition, paxilline is a "competitive" inhibitor with respect to high concentrations of ATP, increasing the regulatory binding site K(m), without affecting the catalytic binding site K(m). At higher concentrations, paxilline inhibits phosphoenzyme formation from ATP and inorganic phosphate, without affecting nucleotide binding. We therefore suggest that paxilline has two effects on the Ca2+ ATPase. At lower concentrations (5-10 microM), paxilline inhibits the ATP-dependent acceleration of Ca2+ release from the phosphoenzyme and/or phosphoenzyme decay. At higher concentrations, paxilline inhibits phosphoenzyme formation.