Concentrations of atracurium and laudanosine in cerebrospinal fluid (CSF) and plasma were assayed in nine patients receiving atracurium infusions of 111-251 min duration to maintain neuromuscular block during intracranial surgery. The total dose of atracurium was 1.57-2.60 mg kg-1 and the plasma concentration of atracurium was 1.27-5.44 micrograms ml-1. Concentrations of laudanosine in CSF and plasma increased during the infusion period, and after 125-140 min reached means of 202.5 ng ml-1 and 1448.7 ng ml-1, respectively. The highest recorded concentration of laudanosine in CSF was 570 ng ml-1, in one of two CSF samples found to contain atracurium. After operation, two patients had fits, but these were not thought to be related to laudanosine. It is concluded that during infusion of atracurium, laudanosine accumulates in both plasma and CSF.