The cortex contains multiple cell types, but studies of attention have not distinguished between them, limiting understanding of the local circuits that transform attentional feedback into improved visual processing. Parvalbumin-expressing inhibitory interneurons can be distinguished from pyramidal neurons based on their briefer action potential durations. We recorded neurons in area V4 as monkeys performed an attention-demanding task. We find that the distribution of action potential durations is strongly bimodal. Neurons with narrow action potentials have higher firing rates and larger attention-dependent increases in absolute firing rate than neurons with broad action potentials. The percentage increase in response is similar across the two classes. We also find evidence that attention increases the reliability of the neuronal response. This modulation is more than two-fold stronger among putative interneurons. These findings lead to the surprising conclusion that the strongest attentional modulation occurs among local interneurons that do not transmit signals between areas.