Motivation is usually inferred from the likelihood or the intensity with which behavior is carried out. It is sensitive to external factors (e.g., the identity, amount and timing of a rewarding outcome) and internal factors (e.g., hunger or thirst). We trained macaque monkeys to perform a non-choice instrumental task (a sequential red-green color discrimination) while manipulating two external factors, reward size and delay-to-reward. We also inferred the state of one internal factor, level of satiation, by monitoring the accumulated reward. A visual cue indicated the forthcoming reward size and delay-to-reward in each trial. The fraction of trials completed correctly by the monkeys increased linearly with reward size and was hyperbolically discounted by delay-to-reward duration, relations that are similar to those found in free operant and choice tasks. The fraction of correct trials also decreased progressively as a function of the satiation level. Similar (albeit more noisy) relations were obtained for reaction times. The combined effect of reward size, delay-to-reward, and satiation level on the proportion of correct trials is well described as a multiplication of the effects of the single factors when each factor is examined alone. These results provide a quantitative account of the interaction of external and internal factors on instrumental behavior, and allow us to extend the concept of 'subjective value' of a rewarding outcome, usually confined to external factors, to account also for slow changes in the internal drive of the subject.