Urine levels of free and total morphine were determined by GC/MS for four male subjects who received single doses of 20 mg of morphine sulfate intramuscularly. Peak concentrations were observed within 10 h for both conjugated and free morphine; thereafter, levels declined rapidly. Initially, free morphine represented from 25 to 34% of the total amount of morphine present, but this ratio declined after 12 h to an average of only 5.9% of total morphine. Free morphine accounted for an overall mean of 6.8% of the dose excreted in urine and conjugated morphine for 58.6%. The mean excretion half-life for free morphine was 6.6 h and for conjugated morphine was 8.2 h. The lower concentration and shorter half-life of free morphine resulted in a shorter detection time for free morphine versus total morphine at a 300-ng/mL cutoff. An equivalent detection time for free morphine was obtained when its cutoff was lowered to 25 ng/mL. The possibility that morphine is metabolized to codeine was unequivocally ruled out by the finding of an absence of codeine at or above the LOD of the GC/MS assay in all clinical specimens collected after morphine administration.