Time plays an essential role in reward valuation. While animals prefer large rewards to small rewards, the preference reverses if the large reward is delayed too long. This thesis is divided into four chapters that explore this curtailing effect of delay on reward valuation. Chapter 1 introduces delay discounting and neural models of interval timing to provide context for the questions that are asked in this thesis. In Chapter 2, we ask whether repeated treatment with the psychostimulant methylphenidate alters delay discounting in rats in the long run and whether any such effect is contingent on exposure at certain developmental stages. Compared with controls, rats treated with moderate doses of methylphenidate during early adolescence showed a long-term increase in delay discounting, indicating they had become impulsive. This brought up the question of what the drug was affecting: the magnitude of the reward, the perception of delay or how they combine. To gain insight, some of the more rudimentary assumptions about the combination of subjective reward and subjective delay needed to be confirmed. Chapter 3 addresses one of those assumptions: whether the computation of reward magnitude is independent from reward delay. We used intracranial self-stimulation and the reward-mountain model to tackle this question. The reward-mountain model describes how various dimensions of value combine. Through the model, we can computationally derive whether manipulations change the reward sensitivity by affecting the directly stimulated neurons or their integration, or whether they change the output of the integration. In contrast to the common assumption that relative subjective reward intensity and reward delay are computed independently and combined in a simple multiplicative manner, we show that delay affected reward processing at multiple stages during the computation of reward value. When reward was delayed by 2 s and 4 s, not only were later stages of the reward circuitry affected, but a reduction in reward sensitivity was also observed at early stages of processing, at the directly stimulated neurons or their integration. The final chapter discusses the implications of these results, including the possibility that the neural signal for reward magnitude and its delay may be interwoven.