Two anterior pituitary homografts were inserted under the left kidney capsule of adult male Sprague-Dawley rats (experimental rats). Sham-treated rats were exposed to identical surgical manipulations, but no tissue was inserted under the kidney capsule. The concentrations of dopamine (DA) and noradrenaline (NA) were determined in the median eminence, intermedioposterior pituitary and telencephalon by a radioisotope-enzyme technique. The decline in DA or NA 2 h after treatment with α-methyl-β-tyrosine provided a measure of DA and NA turnover, respectively. A marked increase in DA turnover was observed in the median eminence of the experimental compared to the sham-treated rats at 1, 3 and 6 days following surgery. The concentrations of DA and of NA were significantly reduced in the median eminence of the experimental rats compared to sham-treated rats at both 3 and 6 days following surgery. No statistically significant increases in DA or NA turnover were observed in either the intermedioposterior lobe of the pituitary or the telencephalon of the experimental animals compared to sham-treated animals at any time investigated. Serum prolactin (PRL) levels were significantly elevated in the experimental animals compared to the sham-treated animals by 3 days following surgery. The results of these studies clearly show that the turnover of DA in the terminals of the tuberoinfundibular neurons is increased by 1 day following the transplantation of anterior pituitary homografts under the kidney capsule. Future studies will be required to determine if this response is due exclusively to an early increase in serum PRL level