The effects of various treatments on the viscosity of the deep dermis of the aspidochirote sea cucumber Actinopyga agassizi were investigated to elucidate the physiological mechanisms of viscosity regulation. Dermal specimens showed a significantly reduced viscosity in the presence of the Ca 2+ chelator EGTA, the Ca 2+ antagonists verapamil and TMB-8, isobutylmethyl xanthine, propranalol, phentolamine, caffeine and 1,9-dideoxyforskolin. With the exception of the Ca 2+ chelator, these drugs were used in the presence of normal extracellular Ca 2+ concentrations. The viscosity of the dermis was significantly increased by 100 mM K + and by phorbol myristate acetate. These compounds were ineffective in the presence of EGTA. Inconsistent effects were seen with cyclic AMP, 8-bromo-cyclic AMP, ATP, sodium orthovanadate, staurosporine, forskolin, trifluoperazine, atropine, isoproterenol and acetylcholine. No effects were seen with okadaic acid, genistein, H-7, hypericin and epinephrine. Lysis of cellular membranes, caused either by freezing and thawing the tissue or by exposing it to the non-ionic detergent Triton X-100 (1%) or to deionized water, significantly increased the viscosity, even in the continuous presence of EGTA. Extracts of frozen and thawed dermis contained one or more non-dialyzable, heat labile factors that stiffened the dermis in the presence of EGTA. The results are consistent with the notion that stiffening of the dermis is the result of a Ca 2+-dependent secretory event that releases a soluble organic stiffening factor into the dermis.