Percutaneous absorption of theophylline in human skin from five sources was examined by use of a flow-through in vitro diffusion system. The metabolites and unchanged drug were estimated by thin-layer chromatography. Correlation was evident in the percentage of the applied dose that diffused through the five skin samples (range 2.8 +/- 0.5%-7.7 +/- 0.8%); however, the percentage of applied dose absorbed varied between different skin samples (range 3.6 +/- 0.9%-33.4 +/- 2.4%). Between 0.2 +/- 0.1%-4.6 +/- 0.2% of the doses applied were metabolized, and over 60% of the total metabolites formed diffused through the skin. The uptake and metabolism of theophylline by microsomes obtained from four of the human skin samples were measured. All preparations showed detectable activities for the metabolism of theophylline. Microsomal preparations from skin sources A, B, and E, and B, C, and E biotransformed theophylline to 1,3,7-trimethyluric acid and 1,3-dimethyluric acid, respectively. The activities of microsomes from skin samples C and E on the drug produced the pharmacologically active metabolite 3-methylxanthine. The specific activities of the microsomes from skin sources A-E for the formation of 1,3-dimethyluric acid and 3-methylxanthine varied fivefold. However, the variation in specific activities of the microsomes for the formation of 1,3,7-trimethyluric acid was twofold (range 2.8 +/- 0.1-6.2 +/- 0.5 pmol/min per mg protein). These metabolic data may be of value in the development of transdermal theophylline systems. The results indicate that a high level of absorption enhancement will be required before transdermal theophylline preparations could produce therapeutic plasma concentrations.