The ontogenesis of rat forebrain adenosine uptake sites labelled by [3H]nitrobenzylthioinosine ([3H]NBI) was determined and compared to that of rat forebrain adenosine receptors labelled by N6-cyclohexyl[3H]adenosine ([3H]-CHA). [3H]NBI binding is highly invariant with similar levels of [3H]NBI binding sites from embryonic day 19 to day 30 postpartum. Scatchard and Hill analyses reveal the binding of [3H]NBI in 6-day-old tissue to be indistinguishable from such binding in 30-day-old tissue. In contrast, [3H]-CHA binding is highly variant. [3H]CHA binding develops slowly but steadily from about embryonic day 19, with adult binding levels being achieved at around 25 days postpartum. The ontogenetic profile of [3H]CHA appears to coincide with synaptogenesis whereas that of [3H]NBI does not.