GTPase activity was studied in plasma membranes purified from the clonal beta-cell line RINm5F. GTPase activities were identified as two broad classes with high or low affinity for GTP. The low-affinity GTPase activity had a Km > 60 microM. In contrast, the high-affinity activity had a Km of 225 nM. Only the high-affinity activity was stimulated by galanin. The stimulated activity had a higher Km (448 nM) and Vmax (75 pmol P(i).min-1.mg-1 protein) compared with the basal. This does not necessarily reflect a complex mechanism of stimulation. Rather, it may reflect that basal activity most likely results from multiple GTPases, whereas the stimulated activity probably reflects one or two specific GTPases. Galanin stimulated the high-affinity GTPase, over the concentration range in which it inhibits stimulated insulin secretion, to a maximal rate 80% greater than the basal rate. The EC50 was 5 nM. Murine and porcine galanin had similar potencies and intrinsic activities on the GTPase. Treatment of the RINm5F cells with PTX before making membranes completely eliminated the stimulatory effect of galanin. Thus, galanin stimulates PTX-sensitive GTPase activity in RINm5F cell membranes in a manner consistent with receptor activation.