The effects of whole drilling muds on the normal activity of walking leg chemosensory neurons were examined using extracellular neurophysiological recording techniques. Exposure of legs for 3-5 min to 10-mg/L drilling mud suspended in sea water altered responses to food odors of 29% of the chemoreceptors examined (data polled for the two drilling muds tested); similar exposure to 100-mg/L drilling mud resulted in interference with 44% of all receptors studies. The effects of both of these concentrations are statistically significant, although they are not different from each other. Interference was usually manifested as a marked reduction in the number of action potentials in a response. In one preparation, the exposure to drilling mud caused a change in the temporal pattern of the spikes without affecting the total number of spikes. Other chemosensory neurons were excited by 10-mg/L drilling mud itself. However, not all chemoreceptors are affected by these drilling muds since responses to feeding stimuli were recorded from the legs of lobsters that had been exposed to drilling mud for 4-8 d before the neurophysiological experiments.