Insulin delivery by somatic cell gene therapy was evaluated using murine pituitary AtT20MtIns-1.4 cells. These cells have been stably transfected to release human insulin by the introduction of a recombinant plasmid bearing a human preproinsulin cDNA under the control of a zinc-sensitive metallothionein promoter. 6 x 10(7) AtT20MtIns-1.4 cells were implanted subcutaneously into streptozotocin-diabetic mice immunosuppressed with cyclosporin A. Release of human insulin was assessed using a specific plasma human C-peptide assay. On days 1 and 2 after implantation human C-peptide concentrations were about 0.02 pmol/ml. Consumption of zinc sulphate solution (500 mg/l) as drinking fluid for days 3-5 increased plasma human C-peptide concentrations to 0.11 +/- 0.01 pmol/ml (mean +/- S.E.M.), n = 11, P < 0.01, and concentrations declined when zinc was discontinued. The extent of hyperglycaemia was slightly lower (P < 0.05) than in a group implanted with non-transfected AtT20 cells. The study was terminated after 9 days, and tumour-like aggregations of implanted cells were identified at autopsy. These comprised a large necrotic core with insulin-containing cells at the periphery. The study provides support for the view that somatic cell gene therapy offers a potential approach to insulin delivery in diabetes mellitus.