Serum concentrations of clopenthixol and flupenthixol have been determined during a four weeks dosage interval in patients trelated with intramuscular injections of clopenthixol decanoate or flupenthixol palmitate in Viscoleo®. Maximal drug levels were attained by the end of the first week after injection of either preparation. A period of exponential decline was then recorded. The half‐lives were estimated to 19 days for clopenthixol and 17 days for flupenthixol. These half‐lives most likely refer to the rate of release from the oil depot and not to elimination of drug. The mean ratio between maximal and minimal drug levels was 2.5 for clopenthixol and 3.7 for flupenthixol. Systemic clearance was estimated to about 0.7/min and 0.5/min, respectively.Significant correlation was found between the administered doses and recorded serum levels, between doses and estimated areas under the serum concentration curves, and also between areas and drug levels. The data indicate more limited individual variability than that seen with other psychotropic drugs, given orally.The study clearly demonstrates, that significant serum levels of active drug is maintained throughout the dosage interval by intramuscular injection of clopenthixol decanoate or flupenthixol palmitate in VisColed® every fourth