The kinetics of iodide efflux were studied in four benign thyroid nodules and one carcinoma by observing the fractional rate of radioiodide release from slices prelabeled in vitro. The thyroid:medium ratio for iodide (T:M) of the benign nodular tissue was 1.9 +/- 0.3 (mean +/- SE) with a value of 9.1 +/- 3.3 obtained from adjacent normal tissue. Despite the reduction in iodide concentrating activity, the rate constant for iodide efflux (ko) from the nonfunctioning, benign thyroid nodular tissue (0.112 +/- 0.044 min-1) was not significantly different as compared to normal tissue (0.102 +/- 0.033 min-1). In a papillary carcinoma, the efflux of radioactive iodide appeared markedly enhanced. These studies indicate that in non-functioning, benign thyroid adenomas, the reduction in iodide concentrating activity is not the consequence of an increased iodide efflux.