This study has characterized constrictions of small cutaneous arteries in the guinea pig ear in response to electrical stimulation of the cervical sympathetic nerve (SNS) in vivo. Video microscopy and on-line image analysis were used to examine diameter changes of ear arteries (80-140 µm resting diameter) in anesthetized guinea pigs. Trains of 50-300 impulses, but not single pulses or short trains, produced frequency-dependent (2-20 Hz) constrictions. The purinoceptor antagonist suramin (30 µM) greatly reduced constrictions produced by exogenous ATP but did not affect constrictions produced by SNS at 10 Hz or exogenous norepinephrine. The ₂-adrenoceptor antagonist yohimbine (1 µM) enhanced the peak amplitude of sympathetic constrictions at lower stimulation frequencies (1-5 Hz). The amplitude of constrictions to SNS at 10 Hz was reduced, and the latency of constrictions was increased by the ₁-adrenoceptor antagonist prazosin (1 µM). Constrictions to SNS at 10 Hz remaining after prazosin treatment were reduced in amplitude by dihydroergotamine (2 µM) and were attenuated further by the neuropeptide Y Y₁-receptor antagonist 1229U91 (0.3 µM). Thus norepinephrine and neuropeptide Y act as cotransmitters to mediate sympathetic constriction of small ear arteries at higher stimulation frequencies (10 Hz), but ATP does not seem to contribute directly to these constrictions.