The effect of stable adenosine analogues, including adenosine 5'-N-ethylcarboxamide (NECA) and N6-L-phenylisopropyl-adenosine (L-PIA), were studied on cyclic adenosine 3', 5'-monophosphate (cyclic AMP) accumulation in rat and guinea-pig thymocytes. NECA was approximately 10 times more potent than L-PIA, in thymocytes from both species. D-PIA was more potent in guinea-pig than in rat thymocytes. The effect of a number of adenosine analogues followed the order: NECA greater than 2-chloro-adenosine greater than L-PIA greater than N6-cyclohexyl-adenosine (CHA), an order of potency characteristic for adenosine receptors of the A2-subtype. Thymocytes may be used as a model system to study the pharmacology of such receptors. Several xanthines were studied as antagonists of the NECA (1 microM)-induced cyclic AMP accumulation. The order of potency was: 1,3-diethyl-8-phenylxanthine greater than 8-phenyl-theophylline greater than IBMX = 8-p-sulphophenyltheophylline = verrophylline greater than theophylline greater than caffeine greater than enprofylline greater than theobromine greater than pentoxiphylline. The pA2 value for 8-phenyltheophylline was 0.35 microM, and the antagonism was shown to be competitive. The order of potency of the xanthine is virtually identical to that found earlier in several other systems in which the receptors are of the A1-subtype. None of the xanthine derivatives tested thus seem to discriminate between A1 and A2-receptor-mediated adenosine actions.