Pilocarpine nitrate is a selective M3-type muscarinic acetylcholine receptor (M3 muscarinic receptor) agonist. IC50 & Target: M3 muscarinic receptor^[1] In Vitro: To evaluate the cytotoxicity of Pilocarpine, the morphology and viability of human corneal stromal (HCS) cells are examined by light microscopy and MTT assay, respectively. Morphological observations show that HCS cells exposed to Pilocarpine at a concentration from 0.625 to 20 g/L exhibit dose- and time-dependent proliferation retardation and morphological abnormality such as cellular shrinkage, cytoplasmic vacuolation, detachment from culture matrix, and eventually death, while no obvious difference is observed between those exposed to Pilocarpine below the concentration of 0.625 g/L and controls. Results of MTT assay reveal that the cell viability of HCS cells decrease with time and concentration after exposing to Pilocarpine above the concentration of 0.625 g/L (P<0.01 or 0.05), while that of HCS cells treated with Pilocarpine below the concentration of 0.625 g/L show no significant difference to controls^[2]. The partial muscarinic agonist, Pilocarpine, evokes concentration-dependent relaxation with an EC₅₀ of 2.4 mM in isolated segments of rat tail artery that were constricted with Penylephrine (10 to 200 nM)^[3]. In Vivo: The Pilocarpine-induced saliva secretion of the control rats (CN) and exercised (EX) rats is examined. A significantly greater amount of saliva is induced by Pilocarpine in the EX rats than in the CN rats (P<0.01). Conversely, the Na⁺ concentration in the saliva of the EX rats is significantly lower than that of the CN rats (P<0.05)^[1].