The significance of plasma cholinesterase (pChe) activity for the duration of action of mivacurium in phenotypically normal patients was evaluated in 35 patients during neurolept anaesthesia. The response to train‐of‐four nerve stimulation was recorded using a Myograph 2000. Ten patients with normal pChe (Group 1) and five patients with decreased pChe activity (Group 2) were given a small test dose of mivacurium 0.03 mg kg^‐1. Mivacurium 0.1 mg kg^‐1 was administered following spontaneous recovery from the first dose. The mean suppression of the height of the first (T₁) of the train‐of‐four responses following mivacurium 0.03 mg kg^‐1 patients with normal and decreased enzyme activity was 40% and 56%, respectively, and the mean T₁ suppression after mivacurium 0.1 mg kg^‐1 was 100% in both groups. The times to different levels of twitch height recovery following the 0.1 mg kg^‐1 dose did not differ between the two groups of patients. Another 20 patients with normal or decreased pChe activity (Group 3) were given mivacurium 0.2 mg kg^‐1. In this group the time to maximum block was 1.4 min (1.0–4.0) mean (range) and the time to reappearance of the T₁ response was 15.0 min (7.4–22.7) (range). An inverse relationship was found between the patients' pChe activity and the time to first response. It is concluded that mivacurium is short‐acting in patients with normal pChe phenotype and normal to low‐normal pChe activity. No patient with very low pChe activity was included in the study. A prolonged response to mivacurium may, however, be expected in these patients.