The activation of adenylate cyclase by TSH and thyroid-stimulating immunoglobulins (TSIg) was investigated in membranes prepared from non-toxic goitres. The assay conditions for maximal adenylate cyclase stimulation by TSH in the absence and presence of a cell cytosol preparation were determined. Cell cytosol had no detectable effect on the characteristics of activation by TSH although it increased the production of cyclic AMP in response to the hormone. In the presence of cell cytosol, membrane protein, pH and substrate concentration dependency of adenylate cyclase activation by both TSH and TSIg were similar. In a comparison of the time-courses of activation there was no apparent lag phase in the activation of adenylate cyclase by TSIg but the onset of activation was slower and linearity persisted longer than with TSH.