The neuropeptides neurotensin, substance P, neurokinin-alpha (substance K), and met-enkephalin are present endogenously in the ventral tegmental area (VTA), site of the A10 dopaminergic (DA) cell bodies. In the present study these four peptides were injected bilaterally into the VTA in the rat, and the effects on operant behavior were assessed. Cannulae aimed at the VTA were implanted in four groups of animals, which had been trained to bar-press for food reward on a fixed-interval, 40-s schedule. A fifth group, in which the effects of systemically administered amphetamine were assessed, was also tested. Response rate across the interval was measured, and the index of quarter-life was taken as an indication of the temporal pattern of responding. In addition, a rate-dependency analysis was carried out for all data. Neurotensin (NT, 0.0175, 0.175, 0.5 micrograms in 1 microliter) dose-dependently decreased response rates without affecting quarter-life, and reduced the number of reinforcements obtained. Substance P (SP, 0.1, 1.0, 3.0 micrograms) did not affect responding, and neurokinin-alpha (NKA, 0.1, 1.0, 3.0 micrograms) induced a small increase in responding. Quarter-life was not affected by SP or NKA, but responding on the non-reinforced lever was significantly increased by both peptides. d-Ala-met-enkephalin (DALA, 0.01, 0.1, 1.0 micrograms) induced a dose-dependent increase in responding which was also rate-dependent, and reduced quarter-life. DALA effects were similar to the classic pattern of responding observed after systemic amphetamine. These results suggest that although all these peptides elicit behavioral activation and may affect DA neuronal activity, the behavioral responses can be differentiated with respect to operant behavior.