Detection of anabolic steroids in hair samples has been possible only in fatal cases or in cases of high-continuous dosages. In order to verify the possibility of detecting an acute administration, a sensitive and specific assay has been developed for the simultaneous determination of testosterone, nandrolone and some of their esters in hair. The analytes were extracted from finely cut hair with methanol-trifluoroacetic acid overnight. After the incubation, the mixture was evaporated to dryness, redissolved and extracted with hexane. The dried organic layer was silanised and analysed by GC-MS and GC-MS-MS. A sensitivity of at least 20 pg injected was obtained for all the analytes. In guinea pigs treated with a single intramuscular dose of 10 mg/kg nandrolone decanoate, neither nandrolone decanoate nor nandrolone were found in hair collected after 13 days, while both compounds were clearly detectable after four repeated doses (each dose every 3-4 days) of 20 mg/kg nandrolone decanoate. Neither nandrolone decanoate nor nandrolone could be detected in hair from a male healthy volunteer 1 month after treatment with 50 mg nandrolone decanoate, while his urine still tested highly positive for the main nandrolone metabolite (> 100 ng/ml). Testosterone esters could not be detected in hair of healthy subjects collected respectively 3, 2 and 1 month after a single intramuscular administration of 250 mg testosterone enanthate (five subjects), a single intramuscular coadministration of 25 mg testosterone propionate plus 110 mg testosterone enanthate (one subject), or a single oral administration of 120 mg testosterone undecanoate (three subjects). Otherwise, hair analysis revealed an increase of testosterone concentration corresponding to the period of treatment. Analysis of blood and urine samples confirmed the absorption of those compounds. At the sensitivity achieved by the present method, no detection of nandrolone, nandrolone decanoate nor testosteron esters in hair seems to be obvious after a single dose administration.