first-and third-stage larvae of parelaphostrongylus tenuis, p. andersoni and elaphostrongylus cervi could not be distinguished morphometrically. third-stage larvae of p. tenuis did develop into recognizable adults in experimentally-infected guinea pigs, but p. andersoni and e. cervi did not. at present, elaphostrongyline larvae found in the feces of cervids in eastern north america are most likely to be either p. tenuis or p. andersoni-, this study confirms that the experimental infection of guinea pigs will allow this distinction to be made. because e. cervi is currently believed to be restricted to newfoundland, its third-stage larvae are distinguished by geographic origin. as many as 22 migrating p. tenuis larvae were recovered from outside the central nervous system (cns) of each of seven guinea pigs necropsied between 1 and 27 days post-infection (dpi); all resembled third-stage larvae digested from snails and used for infections. from one to six developing p. tenuis larvae were recovered from the cns of each of fourteen infected guinea pigs. these included third-stage larvae, longer than reported in published literature, which were recovered as early as 9 dpi and as late as 20 dpi in the cns. fourth-stage larvae were recovered between 18 and 47 dpi, and fifth-stage larvae between 20 and 61 dpi; these stages were only found in the cns. the morphology of the buccal capsule and the tail distinguished the third-, fourth and fifth- stage larvae. descriptions and dimensions of third- and fourth-stage p. tenuis larvae recovered from the cns are provided for the first time. lesions found in the stomach wall, mesentery and liver of guinea pigs infected with p. tenuis suggest that third-stage larvae migrate through the abdominal cavity towards the cns. similar lesions found in guinea pigs infected with p. andersoni and e. cervi suggest that larvae of these species follow a similar route. however, p. andersoni apparently failed to reach the musculature of any of six experimentally-infected guinea pigs. similarly, e. cervi was not recovered from the cns or musculature of any of six experimentally-infected guinea pigs. administration of dexamethasone appears to have been ineffective in suppressing the immune response of guinea pigs to infection with these elaphostrongylines. the mean number of p. tenuis larvae recovered from the cns of guinea pigs given dexamethasone was not significantly different from that recovered from guinea pigs given saline. treatment with dexamethasone allowed neither p. andersoni nor e. cervi larvae to establish in guinea pigs.