Single-unit afferent activity was recorded from the hepatic-duodenal nerve of chloralose-anaesthetized sheep using electrophysiological techniques. Impulse activity was examined while the duodenal mucosa was irrigated with different chemical solutions and explored mechanically. Sixty-one mucosal mechanoreceptors were studied and these were divided into three classes of afferent response. (i) Thirty-four units had no background activity. These 'silent' units had a mechanical threshold of 132 mg and had a slowly adapting response to maintained probing. They had non-myelinated axons (mean conduction velocity 0.93 m s-1). (ii) Five units adapted rapidly to mucosal probing. (iii) Twenty-one units had spontaneous background activity. These 'spontaneous' units had a threshold to probing of 750 mg, and a characteristic sustained 'after-discharge' and periods of mechanical insensitivity; 75% of them were non-myelinated (mean conduction velocity 0.73 m s-1). The other units had a mean conduction velocity of 3.3 m s-1. The behaviour of most sensory units remained within these classes with the exception of ten otherwise silent units whose activity became spontaneous after the application of prostaglandin F2 alpha or sodium hydroxide solutions. Eighty per cent of all units were sensitive to a variety of chemical solutions. Five silent units and four spontaneous units were insensitive to the chemical solution applied. The responses to chemical solutions were sustained for up to 90 s but units became refractory to repetitive treatments, often becoming insensitive after the second application. A chemically specific population was subdivided into two groups: subgroup 1, thirty-two units were excited by potassium chloride solutions (five rapid adapting, twenty silent, seven spontaneous); subgroup 2, twelve units were excited by volatile fatty acid solutions (eight silent units, four spontaneous). Three units were excited by both potassium chloride and volatile fatty acid solutions (two silent, one spontaneous). None of the units tested was sensitive to the osmolality of the solutions applied. Units were not excited by thermal changes over the range 10-45 degrees C and all were mechanically sensitive. Thus no chemoreceptor or thermoreceptor with absolute specificity was found. The conclusions of this study are as follows. With five exceptions, all duodenal mucosal units had sustained responses to probing the mucosa. They therefore appear to differ from gastric mucosal units described as 'rapidly adapting mechanoreceptors with chemical sensitivity' (Iggo, 1957).(ABSTRACT TRUNCATED AT 400 WORDS)