The reversibility of inhibition of plasma, red blood cell (RBC), and diaphragm cholinesterase (ChE) and clinical signs in mice given anatoxin-a(s) [antx-a(s)], a ChE inhibitor from Anabaena flos-aquae NRC-525-17, were characterized and compared with the effects of 2 known ChE inhibitors, the organophosphorus compound paraoxon and the carbamate pyridostigmine bromide. To follow recovery of ChE activity, mice were given either a control solution or an LD40 dose of one of the toxicants ip and killed at time points up to 8 d postdosing. After dosing, mice were monitored for diarrhea, fasciculations, respiratory difficulty, salivation, and tremors. In general, clinical signs in mice given antx-a(s) persisted longer than in mice given pyridostigmine and were more similar in duration to the clinical signs in mice given paraoxon. Histologic lesions were not detected in tissues of mice killed after administration of antx-a(s). Anatoxin-a(s) inhibited lesions were diaphragm ChE for greater than 1 but less than 2 d and RBC ChE for 8 d. The time required for recovery from Antx-a(s)-induced inhibition of ChE in plasma, RBC, and diaphragm was similar to or longer than that with paraoxon and longer than that with pyridostigmine. Based on the duration of antx-a(s) induced clinical signs and ChE inhibition in mice, antx-a(s) appears to be an in vivo irreversible inhibitor of ChE.