Our study was designed to explore the possibility of a pharmacokinetic interaction between apomorphine and levodopa/carbidopa (Sinemet®). The hypothesis was raised because we had observed that some dosages of Sinemet® did not induce or induced a delayed ‘on’ period when taken during the active period of a previously administered apomorphine injection. An open, randomised study in a patient with Parkinson’s disease was performed to compare the pharmacokinetic parameters of half a tablet of 25/100 strength Sinemet® taken during the last third of an ‘on’ period of apomorphine 3mg injection with the pharmacokinetic parameters of the same dosage taken after the end of that ‘on’ period. In the former situation, the maximum concentration (Cmax) and the bioavailability of both components of Sinemet® (levodopa and carbidopa) were significatively reduced (p<0.05). The pharmacological mechanism of this interaction is unknown, but it is advisable that each dosage of Sinemet® should be taken after the end of an ‘on’ period induced by apomorphine.