In epididymal mouse spermatozoa, the effects of dibutyryl cyclic adenosine monophosphate 1 mmol/L (dbcAMP), pentoxifylline 5 mmol/L (PX), and/or mastoparan 50 mumol/L (MT) were evaluated for the following parameters: percentage of motile cells and response to hypoosmotic shock (HOS). The gametes were incubated during 80 min (A) or 200 min (B) in Tyrode's medium, and the drugs were added during the last 20 min. In A, dbcAMP + PX (61.5 +/- 5.4%; n = 10) enhanced and MT decreased significantly the population of motile cells (13.4 +/- 5.4%; n = 6) (control: 47.6 +/- 3.9%; n = 11). In B, PX significantly increased this parameter and MT plus PX also exerted a significant detrimental effect. Responses to HOS dropped significantly in the presence of PX + MT in A or in B; in this latter condition a similar decrease was evoked by MT alone. A positive correlation between percentages of swollen and motile spermatozoa was detected in A or in B in samples incubated with PX (r = .58, n = 11 and r = .76, n = 10; p < .05, respectively). These results that support that, in mouse sperm tail, PX would preserve functional membrane integrity, a relevant condition for adequate motility.