1 The effects of intravenous infusions of enprofylline, theophylline, and placebo on subjective ratings and on psychological test performance were studied in a double-blind crossover experiment in 12 healthy subjects who abstained from caffeine throughout the experimental procedures. 2 Mean plasma concentrations of enprofylline were: mean 2.9 mg 1-1 (range 1.9-3.4).Those for theophylline were: mean 12.1 mg 1-1 (range 9.0-14.4).3 Performance on the auditory vigilance task showed a significant improvement with theophylline compared with both enprofylline and placebo. The correct detection rates (out of 90) were 50.3, 43.4 and 39.1 respectively. A similar effect was seen with finger tapping rates: 404, 394 and 390 taps min-' respectively. Other measures showed no significant effects, although choice reaction time showed a trend towards faster responses with theophylline. 4 Subjective ratings showed that subjects were significantly more alert with theophylline than with enprofylline. Subjects reported themselves as significantly more dizzy and ill with both active drugs compared with placebo. 5 These results suggest that emprofylline largely lacks the CNS stimulant effects of theophylline, but that the incidence of other unwanted effects of the drugs may be similar.