Blood cholinesterase determinations were made at weekly or biweekly intervals in 21 children with esotropia who were treated daily with one drop of 0.06% echothiophate iodide for 8 to 12 weeks. Blood cholinesterases fell in all children. This decline began within the first two weeks after treatment was started; the maximum drop occurred within five to seven weeks. After treatment was discontinued it was several weeks before the cholinesterase values reached their pretreatment levels. None of the children in this study developed symptoms of systemic toxicity.