Six patients undergoing diagnostic lumbar myelography were studied with respect to plasma and cerebrospinal fluid (CSF) concentrations of two xanthine drugs—enprofylline and theophylline. Three patients received enprofylline and three patients received theophylline, 200 mg t.i.d., and plasma and CSF were sampled on the morning of the third day of treatment. CSF plasma ratios averaged 0.095 with enprofylline (range of 0.094–0.097) and 0.36 with theophylline (range of 0.35–0.37). The different ratios of the two drugs contrast to their similar plasma protein binding, about 50%. Different lipophilicity or differences with regard to transport out of the CSF may explain the discrepancy.